Add trailing CRLF to multipart.

.NET's MVC 4 fails to parse multipart uploads which lack a trailing CRLF. This is safe to add to all multipart requests since it is after the final boundary which is considered part of the epilogue and is ignored (per RFC1341 section 7.2.1).
Merge pull request #552 from square/jw/wtf-dot-net

Add trailing CRLF to multipart.