Fix hashing problems for Subs with Dummy's (issue 3969, for example)

http://code.google.com/p/sympy/issues/detail?id=3969

It seems, in the current master some tests in test_wester.py/test_gruntz.py use a lot of memory (up to 3Gb+ on my local system, just before dying due to system limits).  I think, that the problem was triggered by gruntz test 8.11, see [this](https://github.com/sympy/sympy/pull/2000#issuecomment-16275493).

After this fix I don't see such huge memory usage locally.