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.
For the purpose of this test, think about just making a controller that returns objectresult with the formatters in the order you want. This isn't really a test of the *options*, it's a test of a specific scenario for formatters.

Suggest moving the test for the output formatter options to a follow up item.