http: fix two Transport gzip+persist crashes
There were a couple issues:
-- HEAD requests were attempting to be ungzipped,
despite having no content. That was fixed in
the previous patch version, but ultimately was
fixed as a result of other refactoring:
-- persist.go's ClientConn "lastbody" field was
remembering the wrong body, since we were
mucking with it later. Instead, ditch
ClientConn's readRes func field and add a new
method passing it in, so we can use a closure
and do all our bodyEOFSignal + gunzip stuff
in one place, simplifying a lot of code and
not requiring messing with ClientConn's innards.
-- closing the gzip reader didn't consume its
contents. if the caller wasn't done reading
all the response body and ClientConn closed it
(thinking it'd move past those bytes in the
TCP stream), it actually wouldn't. so introduce
a new wrapper just for gzip reader to have its
Close method do an ioutil.Discard on its body
first, before the close.
Fixes #1725
Fixes #1804
R=rsc, eivind
CC=golang-dev
https://golang.org/cl/
4523058