Filename mimetype overrides

[HappyHax0r]

I finally got around to doing this change... This allows you to do something akin to this:

    req = Typhoeus::Request.new('http://localhost:8081/',
                            method: :post,
                            body: { 
                                test: 'test', 
                                test2: 'test2',
                                file: { handle: File.open('somefileondisk.bin', 'r'), filename: 'test.png' }
                            },
                            headers: { 'X-Test' => 'None' },
                            params: 'foo=bar',
                            followlocation: :redirects,
                            multipart: true);

req.run

And will still upload the multipart file, but override the filename (and optionally the mimetype using the mimetype: parameter in the hash) from what would normally be pulled from file_info.

This is useful for anyone using the library who might need to control the filename and mimetype of the file being uploaded (we use it for security tests).

[HappyHax0r]

(For the record the old way of using just a filehandle hasn't changed... it's still supported. This hash-in-hash technique was the easiest and least-surgery way to provide filename and mimetype overrides)