Fixed 100-continue issues

* #1346 Fixed race condition when 100 continue is returned
* Fixed incompatibility between rejecting body and keep alive
* Refactored integration tests to have a more complete very low level HTTP Client
* Introduced HTTP pipelining integration tests
Merge pull request #1346 from jroper/1346-100-continue-fix

Race condition in body handling when Expect: 100-continue