Based on this answerthis answer you could try something like this untested code:
add_filter( 'clean_url', 'wpse_77227_decode_googlefont_url' );
function wpse_77227_decode_googlefont_url( $url )
{
$host = parse_url ($url, PHP_URL_HOST );
if ( 'fonts.googleapis.com' === $host )
return urldecode( $url );
return $url;
}