correctly handle POST into cancelXDCR

I accidentally spotted that we're calling non-existent functions in
that code path.

I've also changed UI to do POST to test POST endpoint.

Change-Id: I2de266d2e85e76aecc6f13e3f9689cfedf08a9ea
Reviewed-on: http://review.couchbase.org/35280
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>