gRPC: generate error when response size is wrong.

gRPC: generate error when response size is wrong.

As long as the "Content-Length" header is given, we now make sure
it exactly matches the size of the response.  If it doesn't,
the response is considered malformed and must not be forwarded
(https://tools.ietf.org/html/rfc7540#section-8.1.2.6).  While it
is not really possible to "not forward" the response which is already
being forwarded, we generate an error instead, which is the closest
equivalent.

Previous behaviour was to pass everything to the client, but this
seems to be suboptimal and causes issues (ticket #1695).  Also this
directly contradicts HTTP/2 specification requirements.

Note that the new behaviour for the gRPC proxy is more strict than that
applied in other variants of proxying.  This is intentional, as HTTP/2
specification requires us to do so, while in other types of proxying
malformed responses from backends are well known and historically
tolerated.