OneHotEncoder.get_feature_names doesn't work with integer column names

```python
import pandas as pd
import numpy as np
from sklearn.preprocessing import OneHotEncoder
X = pd.DataFrame({1: np.random.randint(0, 10, size=400)})
OneHotEncoder().fit(X).get_feature_names([1])
```
```pythontb
TypeError: unsupported operand type(s) for +: 'int' and 'str'
```