Bug 1122047 - If 'Accept' header field is not explicitly specified, server
responds with empty body upon REST query metric/data/{scheduleId}/raw

in case accept header does not specify media type, we fallback to JSON.

(cherry picked from commit a0ba10f41ee50096b1285ced75c335e34026ae44)
Signed-off-by: Lukas Krejci <lkrejci@redhat.com>