Fix for #3626

Function._eval_series() does naive and completely wrong prediction for number of taylor terms if the argument of function is non-atomic.

Simple test:
```
In [1]: sin(sqrt(x))._eval_nseries(x, 3, None)
Out[1]: 
         3/2        
  ___   x       ⎛ 3⎞
╲╱ x  - ──── + O⎝x ⎠
         6          
```