http: follow relative redirect in Get
http: follow relative redirect in Get

Fixes issue 1431.

R=bradfitz, dangabrad
CC=golang-dev
http://codereview.appspot.com/4077042

npe9/noahevans-go9,rdp/rogerpack2005-golang,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,scirelli/scirelli-go,d0f/go-zh,scirelli/scirelli-go,rflanagan/reginaldflanagan-project1,npe9/noahevans-go9,scirelli/scirelli-go,Triskite/willstone-goclone,bryanxu/go-zh,rflanagan/reginaldflanagan-project1,Triskite/willstone-goclone,rdp/rogerpack2005-golang,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,Triskite/willstone-goclone,mhennings/marcohennings-go,bryanxu/go-zh,webfd/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,rflanagan/reginaldflanagan-project1,sanjosh/sanjos100-tipc,Triskite/willstone-goclone,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,bryanxu/go-zh,npe9/noahevans-go9,rdp/rogerpack2005-golang,bryanxu/go-zh,npe9/noahevans-go9,sanjosh/sanjos100-tipc,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,rflanagan/reginaldflanagan-project1,rdp/rogerpack2005-golang,bryanxu/go-zh,sanjosh/sanjos100-tipc,npe9/noahevans-go9,d0f/go-zh,rdp/rogerpack2005-golang,rflanagan/reginaldflanagan-project1,bryanxu/go-zh,webfd/go-zh,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,mhennings/marcohennings-go,Triskite/willstone-goclone,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,bryanxu/go-zh,bryanxu/go-zh,Triskite/willstone-goclone,d0f/go-zh,scirelli/scirelli-go,npe9/noahevans-go9,d0f/go-zh,webfd/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,mhennings/marcohennings-go,npe9/noahevans-go9,d0f/go-zh,rflanagan/reginaldflanagan-project1,scirelli/scirelli-go,rflanagan/reginaldflanagan-project1,rflanagan/reginaldflanagan-project1,scirelli/scirelli-go,rdp/rogerpack2005-golang,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,npe9/noahevans-go9,scirelli/scirelli-go,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,Triskite/willstone-goclone,Triskite/willstone-goclone,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go