Don't create Grape::Request multiple times.

[dblock]

For a simple API call we're manufacturing the Grape::Request object many times.

• lib/grape/middleware/formatter.rb:102:in `format_from_extension' (6 times)
• lib/grape/endpoint.rb:380:in `new'
• once if you call .request in the API.

The first one could easily be fixed by storing @request ||= Grape::Request.new, but each middleware and API call would still continue to creating its own request object if it needed one.

This PR attempts to solve that cleanly:

• Remove Base.request.
• Decouple the formatter middleware from using a Grape::Request from the base class, use its own.
• Decouple the oauth2 middleware from using a Grape::Request from the base class, use its own.
• Assign request, headers and params in endpoint on call.

Will leave this open for a bit for some comments, /cc: @mbleigh.

[dblock]

This is somewhat related to #428, the code here could be even simpler (ie. no need to assign @headers and @params and delegate to request.headers and request.params if Grape::Request could memoize those.