Faraday Multipart

The Multipart middleware converts a Faraday::Request#body Hash of
key/value pairs into a multipart form request, but only under these
conditions:

* The request's Content-Type is "multipart/form-data"
* Content-Type is unspecified, AND one of the values in the Body responds to
  #content_type.

Faraday contains a couple helper classes for multipart values:

* Faraday::Multipart::FilePart wraps binary file data with a Content-Type.
  The file data can be specified with a String path to a local file, or an
  IO object.

* Faraday::Multipart::ParamPart wraps a String value with a Content-Type,
  and optionally a Content-ID.