Fixed resolving when Promise is inherited.

Fixed resolving when Promise is inherited.

Previously, njs_promise_resolve() might return njs_object_t instead of
njs_promise_t. Later an instance of njs_object_t was put into a
NJS_PROMISE value. Whereas njs_promise_t is always expected to be inside
of a NJS_PROMISE value.

This closes #813 issue on Github.