MapStack(which extends MapContext) does not implement Map stuff correctly

[Adam Heath]

I haven't had this be a problem for me in production anywhere, but I 
noticed that when remove(Object) is called, it'll remove the value 
from the top-most map in the stack.  This then means any queries for 
the key will fall-thru to lower maps, which is definately *not* what 
should occur.

Additionally, entrySet() returns an unmodifiable Collection of 
Map.Entry.  However, those entries point to the *original* map.  If 
someone called setValue() on the entry, it would modify the original 
map, instead of inserting a value into the top-most map.

If others concur with my reading/understanding, then I'll go about 
adding test cases which expose this, then fixing it.