MB-100 Don't ignore content type in json_request_hilevel.

It's actually used by menelaus_web:handle_join_tail/5.

Change-Id: I4bccfd2cd3763ecba74e3f7a1971fc7320808079
Reviewed-on: http://review.couchbase.org/30790
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>