Fix for #1194 - Error using [HttpPost] and [Route] together

This change enables some compatibility scenarios with MVC 5 by expanding
the set of legal ways to configure attribute routing. Most promiently, the
following example is now legal:

[HttpPost]
[Route("Products")]
public void MyAction() { }

This will define a single action that accepts POST on route "Products".

See the comments in #1194 for a more detailed description of what changed
with more examples.
There's a bunch of deletions here because there used to be two error cases here, and now there's just one.