Fix for issue-1075 - Move CreateSerializer out of the base class

This is a small refactor as a precursor for api-explorer work.

This adds a virtual `CanWriteType` method to `OutputFormatter` which our xml formatters use to determine which types they can and cannot serialize.
```C#
var contentType = Assert.Single(contentTypes);
Assert.Equal("application/json", contentType.RawValue);
```