Refactor and Test Geocoder::Request Inclusion

[alexreisner]

Note that #1041 broke the gem in two different ways (for users of Sinatra, and for users of Ruby <2.1). We should have tests to ensure that this doesn't happen, and testing will probably require putting the include code into a method that can be called from tests.

Testing should cover adding Geocoder::Request to ActionDispatch::Request and Rack::Request, which is done in lib/geocoder/request.rb.