mode() function returns wrong answer when most common element occurs once
Consider the following code:

```
pd.Series([1]).mode()
=> Series([], dtype: int64)
```

Expected value is the same as for:

```
pd.Series([1,1]).mode()
=> 0    1
=> dtype: int64
```

The same problem happens whenever the most common element only occurs once:

```
pd.Series(range(100)).mode()
Series([], dtype: int64)
```
