Fix handling of null/undefined comparisons
Add another undefined test case
Rename method to reflect that it is PlayScript specific
Fix build warning (file listed twice)
Handle null and undefined keys

In ActionScript, using null or undefined as a key converts the value
to the string "null" or "undefined" respectively.
Use more performant check for undefined
Put null/undefined key handling behind a compile flag

Since the special handling of null and undefined key values is an
as3 wart, I'm going to put it behind a `#if !DISABLE_NULL_KEYS` check,
with the as3 behavior defaulting to enabled. This also means null
checks which were removed were added back.

Also removed the FormatKeyForAs call from a few spots where it wasn't
necessary (due to being called earlier in the call stack).
Use AggressiveInlining for FormatKeyForAs
Merge remote-tracking branch 'origin/cbroglie-undefined'
Handle implicit null conversions