Fix integer check; also add column with integer name in test case.
A small embarrassing mistake is corrected here that was introduced in the recently merged #28215 
Looks like this fixes CI on master - running `my pandas` - currently seeing:
```
pandas/core/generic.py:464: error: Non-overlapping identity check (left operand type: "str", right operand type: "Type[int]")
``` thanks for he quick fix @hwalinga 