New feature: Acceptable

[c960657]

Some servers ignore the Accept header. It usually works fine for static files, but not dynamically generated responses.

This PR adds a new feature which compares the response Content-Type header to the request Accept header and generates a synthetic 406 Not Acceptable response if they don't match.

[ixti]

IMO in this case it's better raise UnacceptableError. Or even make that logic configurable:

HTTP.use(acceptable: { mime_type: "...", on_mismatch: :raise })

• mime_type - if request does not have Accept header - it will inject one
• on_mismatch
  • :raise - fails with UnacceptableError
    • UnacceptableError#response - will contain original response
  • :rewrite_status - rewrites response with new status