Fix documentation for appendPartWithFileURL:name:error:.

The documentation said that the values for filename and the
Content-Type header was being obtained from a NSURLResponse,
but since this is a request, no response is available yet.

The documentation has been changed to reflect better where
those values really come from.
Add method to allow custom file name and content type for multiparts.

The previous method was selecting the file name and the MIME type
automatically depending on the given file URL.

The new method allows specifying a custom file name and a custom
MIME type for the those parts.
Merge pull request #806 from drodriguez/gh-800-multipart-custom-filename

Add parts with custom file names and MIME types