net/textproto: reduce allocations in ReadMIMEHeader
ReadMIMEHeader is used by net/http, net/mail, and
mime/multipart.
Don't do so many small allocations. Calculate up front
how much we'll probably need.
benchmark old ns/op new ns/op delta
BenchmarkReadMIMEHeader 8433 7467 -11.45%
benchmark old allocs new allocs delta
BenchmarkReadMIMEHeader 23 14 -39.13%
benchmark old bytes new bytes delta
BenchmarkReadMIMEHeader 1705 1343 -21.23%
R=golang-dev, r, iant, adg
CC=golang-dev
https://golang.org/cl/
8179043