Empty parts can be either of the form:

a) "--separator\r\n", header (w/ trailing 2xCRLF), \r\n "--separator"...
or
b) "--separator\r\n", header (w/ trailing 2xCRLF), "--separator"...

We never handled case b).  In fact the RFC seems kinda vague about
it, but browsers seem to do a), and App Engine's synthetic POST
bodies after blob uploads is of form b).

So handle them both, and add a bunch of tests.

(I can't promise these are the last fixes to multipart, especially
considering its history, but I'm growing increasingly confident at
least, and I've never submitted a multipart CL with known bugs
outstanding, including this time.)

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/6212046