net/http: fix bug in error checking

Thanks to josef86@gmail.com for pointing this out.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/5477092

src/pkg/net/http/transport.go

@@ -544,7 +544,7 @@ func (pc *persistConn) readLoop() {
 				resp.Header.Del("Content-Length")
 				resp.ContentLength = -1
 				gzReader, zerr := gzip.NewReader(resp.Body)
-				if err != nil {
+				if zerr != nil {
 					pc.close()
 					err = zerr
 				} else {