Rect buffer transfers default arguments fix

This fixes issue https://github.com/inducer/pyopencl/issues/253: type error raised when calling enqueue_copy for rectangular transfers with no values given for optional arguments *_pitches.

Patch changes behaviour to accepting None and setting None as default argument for arguments buffer_pitches, host_pitches for rectangular buffer transfers.

The first, simpler commit 50d4ab55 just changes the default argument to an empty tuple, also fixes this issue, but does not restore the ability to accept None.