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