please make this behave the same as regular series that are multi-variate:

```
>>> sin(x) + cos(y)
sin(x) + cos(y)
>>> _.series()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "sympy/core/expr.py", line 2299, in series
    raise ValueError('x must be given for multivariate functions.')
ValueError: x must be given for multivariate functions.
```