client: disable header compression by default

It is known that header compression leads to information leak.
Header compression is still enabled in servers. See the Google
Groups forum below for related discussions.

See:
http://en.wikipedia.org/wiki/CRIME_(security_exploit)
https://code.google.com/p/chromium/issues/detail?id=139744
https://bugzilla.mozilla.org/show_bug.cgi?id=779413
https://groups.google.com/d/msg/spdy-dev/6mVYRv-lbuc/qGcW2ldOpt8J
1.23.0