net/http: reuse textproto.Readers; remove 2 more allocations
net/http: reuse textproto.Readers; remove 2 more allocations

Saves both the textproto.Reader allocation, and its internal
scratch buffer growing.

benchmark                                   old ns/op    new ns/op    delta
BenchmarkServerFakeConnWithKeepAliveLite        10324        10149   -1.70%

benchmark                                  old allocs   new allocs    delta
BenchmarkServerFakeConnWithKeepAliveLite           19           17  -10.53%

benchmark                                   old bytes    new bytes    delta
BenchmarkServerFakeConnWithKeepAliveLite         1559         1492   -4.30%

R=golang-dev, r, gri
CC=golang-dev
https://codereview.appspot.com/8094046

glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,scirelli/scirelli-go,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,bryanxu/go-zh,scirelli/scirelli-go,mhennings/marcohennings-go,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,scirelli/scirelli-go,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,bryanxu/go-zh,scirelli/scirelli-go,webfd/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,scirelli/scirelli-go,sanjosh/sanjos100-tipc,bryanxu/go-zh,mhennings/marcohennings-go,webfd/go-zh,d0f/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,sanjosh/sanjos100-tipc,d0f/go-zh,bryanxu/go-zh,bryanxu/go-zh,bryanxu/go-zh,rdp/rogerpack2005-golang,d0f/go-zh,webfd/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,webfd/go-zh,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,d0f/go-zh,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,rdp/rogerpack2005-golang,scirelli/scirelli-go,rdp/rogerpack2005-golang,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,mhennings/marcohennings-go,webfd/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof