Adds improved Rack::Deflater tests.

* Gets rid of all repetitions in deflater specs via verify helper method.
Adds deflater options to control compression on per-request level.

* Adds :if option which should be given a lambda accepting env, status, headers, and body options.
* When :if evaluates to false a response body won't be compressed.
* Adds :include option which should be given an array of compressible content types.
* When :include don't include request's content type then response body won't be compressed.