http: add Flusher type; remove Flush from ResponseWriter
http: add Flusher type; remove Flush from ResponseWriter

The Flush functionality wasn't removed, but now you have
to test if your ResponseWriter is also a Flusher:

func ServeHTTP(rw http.ResponseWriter, req *http.Request) {
   if f, ok := rw.(http.Flusher); ok {
       f.Flush()
   }
}

R=rsc, bradfitzwork
CC=gburd, golang-dev
http://codereview.appspot.com/4239077

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