rbovirt: Preserve set headers
rbovirt: Preserve set headers

The headers passed into the http_headers method are merged last to
preserve the ones already set.

This preserves the 'detail=X' additions to the :accept header in
OVIRT::Client#vms, which in turn will save a few round trips for machine
details.

pdilung/rbovirt,abenari/rbovirt,pparkkin/rbovirt