ResultSet.union() terms compatibility check is too restrictive

[Celso Providelo]

In order to setup a UNION ResultSet successfully the find_spec in both terms has to be identical and sometimes that's too restrictive. For instance:

{{{
result1 = self.store.find((Foo, SQL('1')), Foo.id == 20)
result2 = self.store.find((Foo, SQL('2')), Foo.id == 30)
result = result1.union(result2)
}}}

For SQL and FuncExpr items, it seems that it would be safe enough to continue even if the they are not identical objects.

[James Henstridge (jamesh)]

If you want the attached branch to be reviewed, please submit a merge proposal.

Taking a quick look at the diff, I have a few comments:

 1. you probably only want to do this sort of check if is_expr1 is True.
 2. it would be better to check that both expressions have the same variable factory (i.e. getattr(info1, "variable_factory", Variable) is the same as getattr(info2, "variable_factory", Variable)).

Special casing SQL and FuncExpr seems wrong.