Regard all modifier keys in SingleKeyAction

26814a1b53fc added META, but COMMAND and LEFT_* were left out as
they were misinterpreted as aliases, which they aren't (they only
use the enum definition to get the same char code of the keys).

This time added a test case to avoid similar issues in the future.

Fixes
https://code.google.com/p/selenium/issues/detail?id=4843