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

Triskite/willstone-goclone,Triskite/willstone-goclone,d0f/go-zh,rdp/rogerpack2005-golang,rflanagan/reginaldflanagan-project1,scirelli/scirelli-go,mhennings/marcohennings-go,scirelli/scirelli-go,rdp/rogerpack2005-golang,scirelli/scirelli-go,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,mhennings/marcohennings-go,rflanagan/reginaldflanagan-project1,scirelli/scirelli-go,webfd/go-zh,scirelli/scirelli-go,mhennings/marcohennings-go,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,webfd/go-zh,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,Triskite/willstone-goclone,bryanxu/go-zh,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,webfd/go-zh,Triskite/willstone-goclone,webfd/go-zh,sanjosh/sanjos100-tipc,rflanagan/reginaldflanagan-project1,webfd/go-zh,Triskite/willstone-goclone,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,rflanagan/reginaldflanagan-project1,Triskite/willstone-goclone,bryanxu/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,rflanagan/reginaldflanagan-project1,Triskite/willstone-goclone,bryanxu/go-zh,bryanxu/go-zh,scirelli/scirelli-go,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,rflanagan/reginaldflanagan-project1,webfd/go-zh,scirelli/scirelli-go,rflanagan/reginaldflanagan-project1,Triskite/willstone-goclone,mhennings/marcohennings-go,mhennings/marcohennings-go,rdp/rogerpack2005-golang,scirelli/scirelli-go,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,d0f/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,d0f/go-zh,mhennings/marcohennings-go,webfd/go-zh,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,bryanxu/go-zh,rflanagan/reginaldflanagan-project1,webfd/go-zh,d0f/go-zh