No headers bodytype

[JackKelly-Bellroy]

Stop BodyTypes incorrectly adding NoContent to the list of body types if it appears within a Headers (on endpoints that set response headers but don't return a response body).

[JackKelly-Bellroy]

servant-openapi3 still asks for ToSchema NoContent if you have a WithStatus s NoContent in your API, but that feels dubious anyway. If we want to fix it, I think we should invert the chaining of instances currently used, where the instance for Verb method status cs a delegates to Verb method status cs (Headers '[] a). If we always peel off layers when we chain (e.g., a new instance HasOpenApi (Verb method status' cs a) => HasOpenApi (Verb method status cs (WithStatus status' a)) , we should be able to avoid having to add special cases for NoContent in multiple places, and have a single overlapping instance to catch Verb method status cs NoContent.

I wanted to check in before doing this (which can happen in a separate PR) in case the current instance layout was for a reason I wasn't aware of.

[tchoutri]

@maksbotan Can you take a look at this please? :)

[michaelwebb76]

Any chance someone could take a look at this when you have a minute? 🙏

[tchoutri]

@maksbotan ping! :)