Fix exception while series expansion of function with multiple args

```
>>> zeta(x, y).series(y, 0, 2)
    Traceback (most recent call last):
      File "<console>", line 1, in <module>
      File "/home/sk/sympy/py3k-sympy/sympy/core/expr.py", line 2360, in
    series
        s1 = self._eval_nseries(x, n=n, logx=None)
      File "/home/sk/sympy/py3k-sympy/sympy/core/function.py", line 502, in
    _eval_nseries
        if (self.func.nargs == 1 and args0[0]) or self.func.nargs > 1:
    TypeError: unorderable types: tuple() > int()
```

An example of this failure (python3 specific) is in PR #1977.