added the $format parameter to the save method

[rez1dent3]

Hello.
I have a problem to save the image in the format I need I have to add EXT to it and then move it.

$filepath = $path . '.jpg';
$image->save($filepath);
copy($filepath, $path);
unlink($filepath);

This PR fix this issue.

[olivervogel]

Thanks. Please write a test for this and add it to the PR. I will merge afterwards.

[rez1dent3]

@olivervogel done. Added one unit test.

[rez1dent3]

@olivervogel When do you plan to release?