Add prefer header

[calebmer]

I'd like to use the Prefer header, could types for this header be added?

[seanmonstar]

This is the first I've heard of this header. Is there a situation where its usage is common?

[calebmer]

If you look at the PostgREST project you'll see it's being used to suppress counts and in a few other cases. The spec also gives some interesting examples. For my usage I'd like to explore the header for styling a response. For example: Prefer: case=camel or Prefer: envelope.

To me, the Prefer header represents the configuration of some niche behaviors.

[calebmer]

Basically it's an optional Expect.