Fix for Content negotiation should fallback to the first formatter that can write the type #1033

Wierd to see these show up on the interface when the only consumer is a `FirstOrDefault`. You could just replace that with `CanWriteResult(context, null)` and then this isn't overspecified.

`null` in that context means "hey, pick whatever you want" - which more accurately describes the semantic we're trying to support. Right now an actual media-type will be passed in, so it's indistinguishable from the normal code path.