fixed bug in object -lookup when fetching value nil or false

Small small bug caused by the fact that if-let would return
the missing value if the value looked up was false or null,
both of which are valid in both js objects and cljs maps.

It was strangely challenging to do the right thing here because
of how hard it is to discern js/undefined from nill/null in
cljs. In the end I just compared the javascript string
representation, which might be the best we can do.

I also left in some code that should be removed but I thought
would be useful for testing. There is a poor man's unit test
which outputs to the js/console the result of 3 lookups.
Before all were returning "missing", but now false and null
are being returned. I thought it was marginally useful, but
wasn't sure where to put it.