Correct minor problem in `CachedDataAnnotationsModelMetadata.ComputeDisplayFormatString()`

- only affects an extreme corner case: user sets `metadata.EditFormatString` then reads
  `metadata.DisplayFormatString`
- an extreme case because `EditFormatString` is normally set only when
  `DisplayFormatString` is set and, if set, it's to the same value
- happened to see this while updating `CachedDataAnnotationsModelMetadata` for this PR

nit: an -> a in an adjacent XML comment in `CachedDataAnnotationsModelMetadata`

src/Microsoft.AspNet.Mvc.ModelBinding/Metadata/CachedDataAnnotationsModelMetadata.cs

@@ -81,19 +81,19 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
         }
 
         /// <summary>
-        /// Calculate <see cref="ModelMetadata.DisplayFormatString"/> based on presence of an
+        /// Calculate <see cref="ModelMetadata.DisplayFormatString"/> based on presence of a
         /// <see cref="DisplayFormatAttribute"/> and its <see cref="DisplayFormatAttribute.DataFormatString"/> value.
         /// </summary>
         /// <returns>
         /// Calculated <see cref="ModelMetadata.DisplayFormatString"/> value.
-        /// <see cref="DisplayFormatAttribute.DataFormatString"/> if an <see cref="DisplayFormatAttribute"/> exists.
+        /// <see cref="DisplayFormatAttribute.DataFormatString"/> if a <see cref="DisplayFormatAttribute"/> exists.
         /// <c>null</c> otherwise.
         /// </returns>
         protected override string ComputeDisplayFormatString()
         {
             return PrototypeCache.DisplayFormat != null
                 ? PrototypeCache.DisplayFormat.DataFormatString
-                : base.ComputeEditFormatString();
+                : base.ComputeDisplayFormatString();
         }
 
         protected override string ComputeDisplayName()
@@ -114,13 +114,13 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
         }
 
         /// <summary>
-        /// Calculate <see cref="ModelMetadata.EditFormatString"/> based on presence of an
+        /// Calculate <see cref="ModelMetadata.EditFormatString"/> based on presence of a
         /// <see cref="DisplayFormatAttribute"/> and its <see cref="DisplayFormatAttribute.ApplyFormatInEditMode"/> and
         /// <see cref="DisplayFormatAttribute.DataFormatString"/> values.
         /// </summary>
         /// <returns>
         /// Calculated <see cref="ModelMetadata.DisplayFormatString"/> value.
-        /// <see cref="DisplayFormatAttribute.DataFormatString"/> if an <see cref="DisplayFormatAttribute"/> exists and
+        /// <see cref="DisplayFormatAttribute.DataFormatString"/> if a <see cref="DisplayFormatAttribute"/> exists and
         /// its <see cref="DisplayFormatAttribute.ApplyFormatInEditMode"/> is <c>true</c>; <c>null</c> otherwise.
         /// </returns>
         /// <remarks>

test/Microsoft.AspNet.Mvc.ModelBinding.Test/Metadata/CachedDataAnnotationsModelMetadataTest.cs

@@ -321,6 +321,44 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             Assert.Equal(expected, result);
         }
 
+        [Fact]
+        private void EditFormatString_DoesNotAffectDisplayFormat()
+        {
+            // Arrange
+            var provider = new DataAnnotationsModelMetadataProvider();
+            var metadata = new CachedDataAnnotationsModelMetadata(
+                provider,
+                containerType: null,
+                modelType: typeof(object),
+                propertyName: null,
+                attributes: Enumerable.Empty<Attribute>());
+
+            // Act
+            metadata.EditFormatString = "custom format";
+
+            // Assert
+            Assert.Null(metadata.DisplayFormatString);
+        }
+
+        [Fact]
+        private void DisplayFormatString_DoesNotAffectEditFormat()
+        {
+            // Arrange
+            var provider = new DataAnnotationsModelMetadataProvider();
+            var metadata = new CachedDataAnnotationsModelMetadata(
+                provider,
+                containerType: null,
+                modelType: typeof(object),
+                propertyName: null,
+                attributes: Enumerable.Empty<Attribute>());
+
+            // Act
+            metadata.DisplayFormatString = "custom format";
+
+            // Assert
+            Assert.Null(metadata.EditFormatString);
+        }
+
         private class DataTypeWithCustomDisplayFormat : DataTypeAttribute
         {
             public DataTypeWithCustomDisplayFormat() : base("Custom datatype")