MB-8120 Allow omitting ramQuotaMB when updating bucket settings.

Change-Id: Ib5fedd1cdef6ba6bb02fd13dcadb280a1bbe92d5
Reviewed-on: http://review.couchbase.org/30080
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>