Add support for FORM POST to HTTP library

[sgjesse]

The server need to provide a way for handling FORM POST. As a FORM POST is special handling of the HTTP request body this should probably a request handler and not built into the HttpRequest object.

This handler should handle both content types:

  application/x-www-form-urlencoded
  multipart/form-data

The _MimeMultipartParser is already in runtime/bin/mime_multipart_parser.dart to parse a multipart/form-data body.

[sgjesse]

Added this to the M1 milestone.

[sgjesse]

Added Duplicate label.
Marked as being merged into #2488.

[kevmoo]

Removed Area-IO label.
Added Area-Library, Library-IO labels.