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