Allow CURRENT_ROLE in GRANTED BY

I was checking some loose ends in SQL conformance, when I noticed: We 
support GRANT role ... GRANTED BY CURRENT_USER, but we don't support 
CURRENT_ROLE in that place, even though in PostgreSQL they are 
equivalent.  Here is a trivial patch to add that.

-- 
Peter Eisentraut              http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services