mime/multipart: report io.EOF correctly on part ending without newlines

mime/multipart: report io.EOF correctly on part ending without newlines

If a part ends with "--boundary--", without a final "\r\n",
that's also a graceful EOF, and we should return io.EOF instead
of the fmt-wrapped io.EOF from bufio.Reader.ReadSlice.

I found this bug parsing an image attachment from gmail.
Minimal test case stripped down from the original
gmail-generated attachment included.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/6118043