fixed rgbToRgbString method that returned "rgb(0,0,0,0)" instead of "rgba(0,0,0,0)" if alpha value was equal to 0
Merge pull request #71 from nikitadyumin/master

[BUG #7931] fixed rgbToRgbString method that returned "rgb(0,0,0,0)" instead of "rgba(0,0,0,0)" if alpha value was equal to 0