Fix clearing wrapper type fields with reflection.

The nullable value type fields already worked, but the use of the CLR property concealed the difference between string and StringWrapper fields.

csharp/src/Google.Protobuf.Test/WellKnownTypes/WrappersTest.cs

@@ -345,5 +345,15 @@ namespace Google.Protobuf.WellKnownTypes
             var message = TestWellKnownTypes.Parser.ParseFrom(stream);
             Assert.AreEqual(6, message.Int32Field);
         }
+
+        [Test]
+        public void ClearWithReflection()
+        {
+            // String and Bytes are the tricky ones here, as the CLR type of the property
+            // is the same between the wrapper and non-wrapper types.
+            var message = new TestWellKnownTypes { StringField = "foo" };
+            TestWellKnownTypes.Descriptor.Fields[TestWellKnownTypes.StringFieldFieldNumber].Accessor.Clear(message);
+            Assert.IsNull(message.StringField);
+        }
     }
 }

csharp/src/Google.Protobuf/Reflection/SingleFieldAccessor.cs

@@ -61,7 +61,7 @@ namespace Google.Protobuf.Reflection
             
             // TODO: Validate that this is a reasonable single field? (Should be a value type, a message type, or string/ByteString.)
             object defaultValue =
-                typeof(IMessage).IsAssignableFrom(clrType) ? null
+                descriptor.FieldType == FieldType.Message ? null
                 : clrType == typeof(string) ? ""
                 : clrType == typeof(ByteString) ? ByteString.Empty
                 : Activator.CreateInstance(clrType);