Fix py2.6 for expect 100 continue

This PR is https://github.com/boto/botocore/pull/303 (which was reverted because it didn't work in py2.6 + the fix for py2.6.

The issue is that py2.6's _send_request is different
than the other supported python versions.  Because of this,
we have to add a check such that we don't send the body if
this response_received flag has been set.

Verified this works on py2.6, py2.7, py3.3, py3.4.  Integration tests passing on all versions.

cc @danielgtaylor