Fix different header length behavior under Varnish 4.
Fix different header length behavior under Varnish 4.

Varnish 4 now returns a 400 error instead of 413 when the header length
is too big.

Also clarify where our header limit actually comes from.

apinf/api-umbrella,OdiloOrg/api-umbrella-router,apinf/api-umbrella,NREL/api-umbrella-router,NREL/api-umbrella-router,OdiloOrg/api-umbrella-router,apinf/api-umbrella,NREL/api-umbrella-router,NREL/api-umbrella,apinf/api-umbrella,NREL/api-umbrella,apinf/api-umbrella,NREL/api-umbrella,NREL/api-umbrella,OdiloOrg/api-umbrella-router