Don't convert CURLFile params to JSON

PHP 5.5 will throw deprecation warnings if users are uploading files with the previously documented `'source' => '@/foo/bar.jpg',` syntax.

The new approach is to use `'source' => new CurlFile('/foo/bar.jpg', 'image/jpeg'),`, which obviously avoids accidental uploads (or upload attempts) from user-provided content which starts with a `"@"` character.