Changes to enable XML Output Formatters.

Functional tests for Output Formatters are not a part of this PR. Will follow up with Functional Tests after @harshgMSFT's conneg check-in.
FYI you can just do this like so:

```
[HttpPost]
public DummyClass GetDummyClass(int sampleInput)
{
    var result = new ObjectResult(new DummyClass { SampleInt = sampleInput });
    result.Formatters.Add(new XmlSerializerOutputFormatter());
    return result;
}
```