Minor refactoring

Fixed a bug where the cache was modified on a ``Table.get``, and removed extraneous conditionals.
I actually prefer the old way (`for query in self._query_cache`), I think it's clearer about what it does. (And by the way it doesn't consume as much memory because it doesn't construct a tuple of all queries and results. But that doesn't really matter for reasonable cache sizes.)
What do you think?