Prevent jsobfu from generating duplicate/reserved tokens.

I got an error from a script that tried to 'set void = 1'.
Add future reserved words.

Gotta stay ahead of the game.
Make random_var_name public, add specs for it.
Remove unneeded quotes.
Add presence spec.
Dropped a space.
Add Rex::RandomIdentifierGenerator.
Kill extra comma.
Oops, $ and _ need to be in the spec.

* Repeats the random check 20 times for each spec.
Use be instead of eq.
Land #3058 - Prevent jsobfu from generating reserved js keywords