http: fix failing Transport HEAD request with gzip-looking response
http: fix failing Transport HEAD request with gzip-looking response

We only want to attempt to un-gzip if there's a body (not in
response to a HEAD)

This was accidentally passing before, but revealed to be broken
when c3c6e72d7cc went in.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/5477093
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go