Avoid uninitialized variable warning, reuse @integration_session.
Refactor the handling of default_url_options in integration tests

This commit improves the handling of default_url_options in integration
tests by making behave closer to how a real application operates.

Specifically the following issues have been addressed:

* Options specified in routes.rb are used (fixes #546)
* Options specified in controllers are used
* Request parameters are recalled correctly
* Tests can override default_url_options directly
Merge pull request #30 from square/backport-integration-fixes

Backport integration fixes