Fix color test

Fix a bug in the PDFColor._is_equals(). Python treats `None == None` as true. 

The problem was if you are using derivative of PDFDraw, and drawing with different colors created by number (but not name), the colors would not change. `PDFDraw._draw_color()` checks if the color has changed using the _is_equals(). But if you started with a color using a number, then tries to switch to another color by number, it is equals says they are, even if the number are different.