Complete the test suite by covering all Steps on Guzzle 4.

- Upgraded send_request feature for 6 HTTP/1.1 verbs.
- Upgraded set_header feature.
- New story: test POST/PUT/PATCH content.
- New story: test POST form data
- New story: test POST raw data.
- Require PHP 5.4 and Guzzle 4.
- With the test app enabled, all tests pass from testapp/.