Added ability to specify the boundary used in a multipart upload request

Now that the boundary is generated randomly, using AFNetworking to upload a pre-made HTTP body from disk (this can affect those that upload in the background using the Background Transfer Service), the boundary generated vs. used in that pre-made HTTP body will differ - causing uploads to fail.

This patch allows for a user to specify a boundary when generating a `multipartFormRequestWithMethod`. The extra parameter is optional and will default to existing behavior if set to `nil`.