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.
I'm not sure. I added a new project and saved the .sln and VS made this. I rebased on yesterday's dev branch. I'll try rebasing today..