Fix T39158: Help > Operator Cheat Sheet causes a python recursive loop.

Core issue exists since ages - the thing you get from bpy.types.YOUR_OT_operator is fuzzy, and may change,
due to the fact that both Operator and OperatorProperties share the same name...
Would be cool to get rid of this issue one day, but for now it's safer to use rna acessor...

This commit is to be backported to 2.70.