By keeping a single copy of the strings that commonly show up
in headers, we can avoid one string allocation per header.

benchmark                  old ns/op    new ns/op    delta
BenchmarkReadMIMEHeader        19590        10824  -44.75%
BenchmarkUncommon               3168         1861  -41.26%

benchmark                 old allocs   new allocs    delta
BenchmarkReadMIMEHeader           32           25  -21.88%
BenchmarkUncommon                  5            5    0.00%

R=bradfitz, golang-dev, dave, rsc, jra
CC=golang-dev
https://golang.org/cl/6721055