Add a workaround to disable chunked transfer-encoding

This is an undocumented workaround to disable chunks when using HTTP/1.1.
It can be used when the client advertises itself as HTTP/1.1 despite not
understanding the chunked transfer-encoding.

Usage can be found looking at the test for it. When activated, Cowboy
will still advertise itself as HTTP/1.1, but will send the body the same
way it would if it was HTTP/1.0.
Merge branch 'adrianroe-streaming-http1.1-compat'