Give ValidationContext.MemberName a distinct value from DisplayName

- #120

`ValidationAttribute` classes that (incorrectly) expected `MemberName` to always be set or that
instantiate `ValidationResult`s using the `DisplayName` may be negatively impacted. For exmaple,
`MemberName` will be `null` when binding parameters and items in a collection.

src/System.Web.Http/Validation/Validators/DataAnnotationsModelValidator.cs

@@ -33,11 +33,11 @@ public override IEnumerable<ModelValidationResult> Validate(ModelMetadata metada
             // Per the WCF RIA Services team, instance can never be null (if you have
             // no parent, you pass yourself for the "instance" parameter).
 
-            string memberName = metadata.GetDisplayName();
+            var memberName = metadata.PropertyName;
             ValidationContext context = new ValidationContext(container ?? metadata.Model)
             {
-                DisplayName = memberName,
-                MemberName = memberName
+                DisplayName = metadata.GetDisplayName(),
+                MemberName = memberName,
             };
 
             ValidationResult result = Attribute.GetValidationResult(metadata.Model, context);

test/System.Web.Http.Test/Validation/Validators/DataAnnotationsModelValidatorTest.cs

@@ -59,7 +59,7 @@ public static TheoryDataSet<ModelMetadata, string> ValidateSetsMemberNamePropert
                     },
                     {
                         _metadataProvider.GetMetadataForType(() => new object(), typeof(SampleModel)),
-                        "SampleModel"
+                        null
                     }
                 };
             }