Sanitize open() method

Removed the mask parameter because a web socket client does not have
a choice between masking and not masking.

Change-Id: I3c33acc235bb2a99476abb6201e6dbb6f881dd70
Reviewed-by: Milian Wolff <milian.wolff@kdab.com>
Reviewed-by: Kurt Pattyn <pattyn.kurt@gmail.com>