wrappercamerabinsrc: Fix caps filter caps proxying

wrappercamerabinsrc: Fix caps filter caps proxying

The notify signal is triggered when caps is changed. But instead of
proxying the fixed caps, we query for the caps. Hence, when we go to
READY state, we endup setting template caps on the proxied caps
filter instead of NULL, which leads to negoitation failure. Correctly
proxy NULL caps if this is the new caps. Fixes not negotiated error
when running in cheese. Also fix a leak of caps string in one of the
trace.

https://bugzilla.gnome.org/show_bug.cgi?id=732741