BUG: Bug in fillna with Series and a passed series/dict (GH5703)
closes #5703

```
In [1]: s1 = Series([np.nan])

In [2]: s2 = Series([1])

In [3]: s1.fillna(s2)
Out[3]: 
0    1
dtype: float64

In [4]: s1.fillna({ 1 : 1})
Out[4]: 
0    1
dtype: float64

In [5]: s1.fillna({ 1 : 2, 2 : 3})

ValueError: cannot fillna on a 1-dim object with more than one value
```

@jreback I think with this change also the docstring should be updated? Because a Series is not mentioned now.
 hmm thought it was
 @jorisvandenbossche docs updated in #5716 
 @jreback Super!
