Respect content-type from provides route annotations

[slobo]

Formatter middleware now respects provides annotation for routes and
uses it both as default and as a restriction on allowed response
content types.

This makes it possible to for ex. output binary blobs on specific paths,
while keeping the rest of the paths as structured data.

To acheive this, we needed to make the following changes:

• Raisin::Middleware::Formatter is now aware of the running Raisin app
  (via raisin attribute)

• ::Formatter->negotiate_format method has gained a new parmeter to
  allow for picking the proper default when provides is configured and
  Accept: */* header is sent

[khrt]

Thanks for the merge request!