Force automatic back-pressure handling by enabling HTTP pipelining

Automatic back-pressure handling has been disabled by default in Spray 1.2.0.

See:
https://groups.google.com/forum/#!topic/spray-user/YN2ocRzwhY0
https://github.com/spray/spray/commit/e8563f7aa694d29e0ebe72e3cf412ff83881ecfe