Remove python25 workaround from glance_http

Since we don't support python25 removing to reduce what to cover in the
unittests.
Also fix a problem with a possible NoneType in the right side of the comparison
cause an exception. Now the default value for content-type header is an empty
string.

Change-Id: I3df5ac75455e62689f08c336fc89a0437dae6945
1 file changed