fix Sum and Product _eval_subs

https://code.google.com/p/sympy/issues/detail?id=3786

```
>>> Integral(x,(x,1,x+y)).subs(x,1)  # CORRECT
Integral(x, (x, 1, y + 1))

>>> Sum(x,(x,1,x+y)).subs(x,1)  # this doesn't replace the x
Sum(x, (x, 1, x + y))

>>> Product(x,(x,1,2))  # raises Traceback
Product(x, (x, 1, 2))
>>> _.subs(x,1)
Traceback (most recent call last):

...
ValueError: Invalid limits given: ((1, 1, 2),)
```

This is a pretty clean fix, making Product and Sum handling in `_eval_subs` consistent with that of Integral. Without this change they are significantly broken in terms of handling of subs.