Check non json type on glance client json_request method

This patch adds Content-Type headers for request and responses in
glance_http json_request methods based on what was discussed in a
previous patch.
(https://review.openstack.org/#/c/84454/4/tempest/tests/test_glance_http.py)
Also includes a couple of unit tests to assure the behavior.

Related to bp unit-tests

Change-Id: I570c86401f878ab8048194be252ce82d1b44f5b4
Merge "Check non json type on glance client json_request method"