Changes to introduce CanWriteResult for Xml OutputFormatters.

Overriding CanWriteResult in XmlOutputFormatters. By doing this, there will never be a case where XmlOutputFormatters are asked to write a type which they do not support.
The media type needs to be a real value. If someone reorders the implementation code to check the media type first, then your test won't test what you think it's testing.