This PR allows for multiple values with the same field name to be sent to an
HTTP server. This server can't be PHP, because PHP's POST processing does not
allow for two the same field names. But sending multiple fields with the same
name is required for some operations by RFC 7578.
This PR allows you to attach an array of values to a field name, and each of
these will then be sent as its own distinct multipart/form-data field.