Feature/add octet stream content type

I know this is not in scope here, but I actually think those mime types aren't truly supported in Grape. They don't automatically map to a formatter, so if you say RSS or Atom or JSONAPI, you're going to get no formatting whatsoever out of your response. We don't even have tests.

If you could remove any reference to those, in this or separate PR, that'd be great. Or at least open an issue. Make sure to add a note to UPGRADING that these were removed and were never properly supported.