Fix #3111 - Don't trounce content-type in VCR

This change makes ViewComponentResult respect an existing Content-Type
setting on the HttpResponse. Code is very similar to the code in
ViewExector, and includes the same quirks.

src/Microsoft.AspNet.Mvc.ViewFeatures/ViewComponentResult.cs

@@ -80,22 +80,32 @@ namespace Microsoft.AspNet.Mvc
                 viewData = new ViewDataDictionary(modelMetadataProvider, context.ModelState);
             }
 
-            var contentType = ContentType ?? ViewExecutor.DefaultContentType;
-            if (contentType.Encoding == null)
+            var contentType = ContentType;
+            if (contentType != null && contentType.Encoding == null)
             {
                 // Do not modify the user supplied content type, so copy it instead
                 contentType = contentType.Copy();
                 contentType.Encoding = Encoding.UTF8;
             }
 
+            // Priority list for setting content-type/encoding:
+            //      1. this.ContentType (set by the user on the result)
+            //      2. response.ContentType (likely set by the user in controller code)
+            //      3. ViewExecutor.DefaultContentType (sensible default)
+            //
+            //
+            response.ContentType = 
+                contentType?.ToString() ?? 
+                response.ContentType ??
+                ViewExecutor.DefaultContentType.ToString();
+
             if (StatusCode != null)
             {
                 response.StatusCode = StatusCode.Value;
             }
 
-            response.ContentType = contentType.ToString();
-
-            using (var writer = new HttpResponseStreamWriter(response.Body, contentType.Encoding))
+            var encoding = contentType?.Encoding ?? ViewExecutor.DefaultContentType?.Encoding;
+            using (var writer = new HttpResponseStreamWriter(response.Body, encoding))
             {
                 var viewContext = new ViewContext(
                     context,

src/Microsoft.AspNet.Mvc.ViewFeatures/ViewFeatures/ViewExecutor.cs

@@ -45,6 +45,10 @@ namespace Microsoft.AspNet.Mvc.ViewFeatures
                 contentType.Encoding = Encoding.UTF8;
             }
 
+            // Priority list for setting content-type:
+            //      1. passed in contentType (likely set by the user on the result)
+            //      2. response.ContentType (likely set by the user in controller code)
+            //      3. ViewExecutor.DefaultContentType (sensible default)
             response.ContentType = contentType?.ToString() ?? response.ContentType ?? DefaultContentType.ToString();
 
             using (var writer = new HttpResponseStreamWriter(response.Body, contentType?.Encoding ?? DefaultContentType.Encoding))

test/Microsoft.AspNet.Mvc.ViewFeatures.Test/ViewComponentResultTest.cs

@@ -309,6 +309,65 @@ namespace Microsoft.AspNet.Mvc
             Assert.Equal(contentTypeBeforeViewResultExecution, contentTypeAfterViewResultExecution);
         }
 
+        [Fact]
+        public async Task ViewComponentResult_SetsContentTypeHeader_OverrideResponseContentType()
+        {
+            // Arrange
+            var descriptor = new ViewComponentDescriptor()
+            {
+                FullName = "Full.Name.Text",
+                ShortName = "Text",
+                Type = typeof(TextViewComponent),
+            };
+
+            var actionContext = CreateActionContext(descriptor);
+
+            var expectedContentType = "text/html; charset=utf-8";
+            actionContext.HttpContext.Response.ContentType = "application/x-will-be-overridden";
+
+            var viewComponentResult = new ViewComponentResult()
+            {
+                Arguments = new object[] { "World!" },
+                ViewComponentName = "Text",
+                ContentType = new MediaTypeHeaderValue("text/html") { Encoding = Encoding.UTF8 },
+            };
+
+            // Act
+            await viewComponentResult.ExecuteResultAsync(actionContext);
+
+            // Assert
+            Assert.Equal(expectedContentType, actionContext.HttpContext.Response.ContentType);
+        }
+
+        [Fact]
+        public async Task ViewComponentResult_NoContentTypeSet_PreservesResponseContentType()
+        {
+            // Arrange
+            var descriptor = new ViewComponentDescriptor()
+            {
+                FullName = "Full.Name.Text",
+                ShortName = "Text",
+                Type = typeof(TextViewComponent),
+            };
+
+            var actionContext = CreateActionContext(descriptor);
+
+            var expectedContentType = "application/x-will-not-be-overridden";
+            actionContext.HttpContext.Response.ContentType = expectedContentType;
+
+            var viewComponentResult = new ViewComponentResult()
+            {
+                Arguments = new object[] { "World!" },
+                ViewComponentName = "Text",
+            };
+
+            // Act
+            await viewComponentResult.ExecuteResultAsync(actionContext);
+
+            // Assert
+            Assert.Equal(expectedContentType, actionContext.HttpContext.Response.ContentType);
+        }
+
         private IServiceCollection CreateServices(params ViewComponentDescriptor[] descriptors)
         {
             var services = new ServiceCollection();