Use string.IsNullOrEmpty instead of string.IsNullOrWhitespace

Fixes #3593

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

@@ -123,7 +123,7 @@ namespace Microsoft.AspNet.Mvc.Formatters
             if (contentType != null)
             {
                 var charset = contentType.Charset;
-                if (!string.IsNullOrWhiteSpace(charset))
+                if (!string.IsNullOrEmpty(charset))
                 {
                     foreach (var supportedEncoding in SupportedEncodings)
                     {

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

@@ -254,7 +254,7 @@ namespace Microsoft.AspNet.Mvc.Formatters
                 for (var i = 0; i < acceptValues.Count; i++)
                 {
                     var charset = acceptValues[i].Value;
-                    if (!string.IsNullOrWhiteSpace(charset))
+                    if (!string.IsNullOrEmpty(charset))
                     {
                         for (var j = 0; j < SupportedEncodings.Count; j++)
                         {

src/Microsoft.AspNet.Mvc.Core/ModelBinding/SimpleTypeModelBinder.cs

@@ -37,7 +37,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
                 {
                     var modelAsString = model as string;
                     if (bindingContext.ModelMetadata.ConvertEmptyStringToNull &&
-                        string.IsNullOrWhiteSpace(modelAsString))
+                        string.IsNullOrEmpty(modelAsString))
                     {
                         model = null;
                     }

src/Microsoft.AspNet.Mvc.TagHelpers/Internal/AttributeMatcher.cs

@@ -100,19 +100,24 @@ namespace Microsoft.AspNet.Mvc.TagHelpers.Internal
             // Perf: Avoid allocating enumerator
             for (var i = 0; i < requiredAttributes.Length; i++)
             {
-                var attribute = requiredAttributes[i];
-                if (!context.AllAttributes.ContainsName(attribute) ||
-                    context.AllAttributes[attribute] == null ||
-                    (context.AllAttributes[attribute].Value is string &&
-                    string.IsNullOrWhiteSpace(context.AllAttributes[attribute].Value as string)))
+                var requiredAttribute = requiredAttributes[i];
+                IReadOnlyTagHelperAttribute attribute;
+                if (!context.AllAttributes.TryGetAttribute(requiredAttribute, out attribute))
                 {
-                    // Missing attribute!
-                    missingAttributes.Add(attribute);
+                    // Missing attribute.
+                    missingAttributes.Add(requiredAttribute);
+                    continue;
                 }
-                else
+
+                var valueAsString = attribute.Value as string;
+                if (valueAsString != null && string.IsNullOrEmpty(valueAsString))
                 {
-                    presentAttributes.Add(attribute);
+                    // Treat attributes with empty values as missing.
+                    missingAttributes.Add(requiredAttribute);
+                    continue;
                 }
+
+                presentAttributes.Add(requiredAttribute);
             }
 
             return new PresentMissingAttributes { Present = presentAttributes, Missing = missingAttributes };

src/Microsoft.AspNet.Mvc.WebApiCompatShim/ContentNegotiator/FormattingUtilities.cs

@@ -183,7 +183,7 @@ namespace System.Net.Http
         /// <returns>Unquoted token.</returns>
         public static string UnquoteToken(string token)
         {
-            if (string.IsNullOrWhiteSpace(token))
+            if (string.IsNullOrEmpty(token))
             {
                 return token;
             }

test/Microsoft.AspNet.Mvc.IntegrationTests/SimpleTypeModelBinderIntegrationTest.cs

@@ -271,7 +271,7 @@ namespace Microsoft.AspNet.Mvc.IntegrationTests
             };
             var operationContext = ModelBindingTestHelper.GetOperationBindingContext(request =>
             {
-                request.QueryString = QueryString.Create("Parameter1", "  ");
+                request.QueryString = QueryString.Create("Parameter1", "");
             });
             var modelState = operationContext.ActionContext.ModelState;
 
@@ -290,10 +290,10 @@ namespace Microsoft.AspNet.Mvc.IntegrationTests
             Assert.False(modelState.IsValid);
             var key = Assert.Single(modelState.Keys);
             Assert.Equal("Parameter1", key);
-            Assert.Equal("  ", modelState[key].AttemptedValue);
-            Assert.Equal("  ", modelState[key].RawValue);
+            Assert.Equal("", modelState[key].AttemptedValue);
+            Assert.Equal("", modelState[key].RawValue);
             var error = Assert.Single(modelState[key].Errors);
-            Assert.Equal("The value '  ' is invalid.", error.ErrorMessage, StringComparer.Ordinal);
+            Assert.Equal("The value '' is invalid.", error.ErrorMessage, StringComparer.Ordinal);
             Assert.Null(error.Exception);
         }