chrome throw a permission denied exception if screen sharing + audio sharing

add a test to remove audio sharing when starting screen sharing. I don't advise to force audio off in `setScreenCapture` since it would be surprising to lose audio when switch back to webcam sharing
Fix typo  isMozillia vs isMozilla
Merge pull request #24 from fbeauchamp/master

Masked out the audio component of a GetUserMedia when doing a screen share.