Fix for UnicodeDecodeError when optimizing multiple files (i.e. when using StringIO) and at least one of the files contains unicode characters

[dnerdy]

Fixed unicode decode error when optimizing combined files that contain unicode

new_file_content.getvalue() returns a str, but codecs.open(new_filepath, 'w', 'utf-8').write() expects unicode, else it b0rks

[peterbe]

Thanks!