builtin tostring blocks metatable __tostring method

I added a check for a metatable to disable the new default.  I figure this is acceptable since, 

If the metatable doesn't have a __tostring method, the new default behavior is still disabled.  I figure this is desirable since it also provides a mechanism to intentionally disable the new default.  Having a default tostring method for tables can obscure errors relating to mismatched types.

Also, in the case that someone has gone through the trouble to add a metatable, adding a __tostring method is trivial.