Fix headers for python-requests-1.2

I didn't find an relevant changelog entry detailing
what may have changed. But from the source code, this
is what seemed to have changed:

headers should be a dict, and not a list.

Works with 1.1 and 1.2 both.
Automatic commit of release 0.15.1rc1
Automatic commit of release 0.15.1
Merge branch 'release-0.15' into develop