Introduce POST size limits for VEP endpoint. Fix configuration issue with check_existing flag being turned on when required and not before. This flag causes a performance hit and is undesirable for default conditions.
Re-write of priority for accept vs. content type.

Catalyst Rest plugin has a known bug concerning the behaviour of Accept
vs. Content-Type
(https://rt.cpan.org/Public/Bug/Display.html?id=46974). It is not
possible to specify a request content-type of say application/json and
set an accept header of text/plain. The plugin will force the output to
be JSON.

This rewrite when we work with POSTs will prioritise the Accept header
over Content-Type. The rewrite does not affect the current behaviour for
GET requests.