Fixed issue with HashWithIndifferentAccess#store_path.

Fixes #2

See #2 <https://github.com/ManageIQ/more_core_extensions/issues/2#issuecomment-23970941>
for a description of the issue.  In essence, `child = self[key] = {}`
when self is a HashWithIndifferentAccess, causes self[key] to ultimately
be set to a different object than child because of the way blah= methods
return values.