Quality method did not work. Now fixed

The quality method was checking if image responded to :quality=, which will always fail. As far as I can see, VIPS::Image contains no quality method (VIPS::JPEGWriter seems to have a quality method, but no quality=). Quality is now set via the writer format options instance variable (@_format_opts). Currently, it assumes that the only format to accept quality is jpeg, checking that either the original file format or conversion format are jpeg before setting the format options variable.