* StringOutput set proper ContentType

src/Microsoft.AspNet.Mvc.Core/Formatters/StringOutputFormatter.cs

@@ -7,7 +7,6 @@ using System.Threading.Tasks;
 using Microsoft.AspNet.Http;
 using Microsoft.AspNet.Mvc.Internal;
 using Microsoft.Extensions.Primitives;
-using Microsoft.Net.Http.Headers;
 
 namespace Microsoft.AspNet.Mvc.Formatters
 {
@@ -34,7 +33,13 @@ namespace Microsoft.AspNet.Mvc.Formatters
             // always return it as a text/plain format.
             if (context.ObjectType == typeof(string) || context.Object is string)
             {
-                context.ContentType = new StringSegment(SupportedMediaTypes[0]);
+                if (!context.ContentType.HasValue)
+                {
+                    var mediaType = SupportedMediaTypes[0];
+                    var encoding = SupportedEncodings[0];
+                    context.ContentType = new StringSegment(MediaType.ReplaceEncoding(mediaType, encoding));
+                }
+
                 return true;
             }
 

test/Microsoft.AspNet.Mvc.Core.Test/Formatters/StringOutputFormatterTests.cs

@@ -27,6 +27,48 @@ namespace Microsoft.AspNet.Mvc.Formatters
             }
         }
 
+        [Fact]
+        public void CanWriteResult_SetsAcceptContentType()
+        {
+            // Arrange
+            var formatter = new StringOutputFormatter();
+            var expectedContentType = new StringSegment("application/json");
+
+            var context = new OutputFormatterWriteContext(
+                new DefaultHttpContext(),
+                new TestHttpResponseStreamWriterFactory().CreateWriter,
+                typeof(string),
+                "Thisisastring");
+            context.ContentType = expectedContentType;
+
+            // Act
+            var result = formatter.CanWriteResult(context);
+
+            // Assert
+            Assert.True(result);
+            Assert.Equal(expectedContentType, context.ContentType);
+        }
+
+        [Fact]
+        public void CanWriteResult_DefaultContentType()
+        {
+            // Arrange
+            var formatter = new StringOutputFormatter();
+
+            var context = new OutputFormatterWriteContext(
+                new DefaultHttpContext(),
+                new TestHttpResponseStreamWriterFactory().CreateWriter,
+                typeof(string),
+                "Thisisastring");
+
+            // Act
+            var result = formatter.CanWriteResult(context);
+
+            // Assert
+            Assert.True(result);
+            Assert.Equal(new StringSegment("text/plain; charset=utf-8"), context.ContentType);
+        }
+
         [Theory]
         [MemberData(nameof(OutputFormatterContextValues))]
         public void CanWriteResult_ReturnsTrueForStringTypes(
@@ -35,9 +77,7 @@ namespace Microsoft.AspNet.Mvc.Formatters
             bool expectedCanWriteResult)
         {
             // Arrange
-            var expectedContentType = expectedCanWriteResult ?
+            var expectedContentType = new StringSegment("application/json");
-                new StringSegment("text/plain") :
-                new StringSegment("application/json");
 
             var formatter = new StringOutputFormatter();
             var type = useDeclaredTypeAsString ? typeof(string) : typeof(object);
@@ -47,7 +87,7 @@ namespace Microsoft.AspNet.Mvc.Formatters
                 new TestHttpResponseStreamWriterFactory().CreateWriter,
                 type,
                 value);
-            context.ContentType = new StringSegment("application/json");
+            context.ContentType = expectedContentType;
 
             // Act
             var result = formatter.CanWriteResult(context);

test/Microsoft.AspNet.Mvc.FunctionalTests/ContentNegotiationTest.cs

@@ -350,7 +350,27 @@ END:VCARD
         [Theory]
         [InlineData(true)]
         [InlineData(false)]
-        public async Task ObjectResult_WithStringReturnType_WritesTextPlainFormat(bool matchFormatterOnObjectType)
+        public async Task ObjectResult_WithStringReturnType_DefaultToTextPlain(bool matchFormatterOnObjectType)
+        {
+            // Arrange
+            var targetUri = "http://localhost/FallbackOnTypeBasedMatch/ReturnString?matchFormatterOnObjectType=true" +
+                matchFormatterOnObjectType;
+            var request = new HttpRequestMessage(HttpMethod.Get, targetUri);
+
+            // Act
+            var response = await Client.SendAsync(request);
+
+            // Assert
+            Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+            Assert.Equal("text/plain", response.Content.Headers.ContentType.MediaType);
+            var actualBody = await response.Content.ReadAsStringAsync();
+            Assert.Equal("Hello World!", actualBody);
+        }
+
+        [Theory]
+        [InlineData(true)]
+        [InlineData(false)]
+        public async Task ObjectResult_WithStringReturnType_SetsMediaTypeToAccept(bool matchFormatterOnObjectType)
         {
             // Arrange
             var targetUri = "http://localhost/FallbackOnTypeBasedMatch/ReturnString?matchFormatterOnObjectType=" +
@@ -363,7 +383,7 @@ END:VCARD
 
             // Assert
             Assert.Equal(HttpStatusCode.OK, response.StatusCode);
-            Assert.Equal("text/plain", response.Content.Headers.ContentType.MediaType);
+            Assert.Equal("application/json", response.Content.Headers.ContentType.MediaType);
             var actualBody = await response.Content.ReadAsStringAsync();
             Assert.Equal("Hello World!", actualBody);
         }