assert triggered in v8 debug build when using node-weak module

[amudukutore]

I see an assertion triggered when running debug version of v8 and trying to get a property
on the weak-reference object via WeakNamedPropertyGetter or WeakIndexedPropertyGetter. I don't have my logs handy but I believe the assertion
was due to failure of:
ASSERT(reinterpret_cast

(*location_) != kHandleZapValue);

Looking at the code, it appears that in these cases, we are failing to return the values through scope.Close().

[TooTallNate]

npm test works for me, as of the nan refactor at least (see #19).