Breaks Camlistore by introducing a datarace. See comments on
https://golang.org/cl/95760043/ for details.

I'll add a new test to lock-in the current behavior in a
subsequent CL.

I don't think Camlistore is particularly unique here: it's doing
the obvious thing to stream a multipart body to a server
using a goroutine feeding the multipart writer.

««« original CL description
mime/multipart: delay reading random source

If a user sets his/her own boundary string with SetBoundary,
we don't need to call randomBoundary at all.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/95760043
»»»

LGTM=ruiu
R=ruiu
CC=golang-codereviews, mathieu.lonjaret
https://golang.org/cl/117600043