Avoid parsing URI multiple times.

isease better than the cure :wink: There is already quite some [duplication in the code](https://codeclimate.com/github/brynary/rack-test/Rack::Test::Session), adding more duplication in the code is imho worse than parsing the URI twice.

Can you find a way to parse it just once and leave the code quality at least at the same level? Extracting `env_for` and and `process_request` into a method should help.