Headers were encoded in HTTPClient, but when glance client started
to use SessionClient this functionality was lost.
This commit replaces static method "encode_headers" from HTTPClient
and makes it a common function, that SessionClient can use when
converting image meta to headers.
Change-Id: If9f8020220d2a0431b4241b38b9c83c09c0d75cb
Closes-bug: #1574587
(cherry picked from commit 9329ef0bc4)