Comment 2 for bug 1634110

Reviewed: https://review.openstack.org/387415
Committed: https://git.openstack.org/cgit/openstack/keystoneauth/commit/?id=ab39cf066784dc01869a7e22a9289e154410e4e5
Submitter: Jenkins
Branch: master

commit ab39cf066784dc01869a7e22a9289e154410e4e5
Author: Mike Fedosin <email address hidden>
Date: Mon Oct 17 15:50:28 2016 +0300

    Prevent changing content type in request

    Now if body contains json, its content type forcibly
    changes to application/json, which is not correct in
    some case, like json-patch request.

    This code fixes this situation and sets application/json
    contnent type only if this header hasn't been set before.

    Change-Id: I4e0c44d444519f056dfa48c9603dbc3ca6b01822
    Closes-bug: #1634110