DataFrame.replace behave strangely
Basically

```
pandas.DataFrame({
    "a": [1,1,1,2,2,2]    
}).replace({
    "a": {
        1: 2,
        2: 3
    }
})
```

returns (I was suprised on that one)

```
    a
0  2
1  3
2  2
3  3
4  2
5  3
```
