BUG: Fix render as column name in to_excel
- [x] closes #34331
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Should I add a whats new enty? Under which section?

https://github.com/phofl/pandas/blob/34331_render_column_name_raises_error_in_to_excel/pandas/io/formats/excel.py#L389

The previous code checked if the `df` had a attribute `render`. This check was True, if there was a `DataFrame` given as input with a column `render`. I think that this check should only be True if a `Styler` object is given, so I added an `isinstace` check for `Styler`. 

Does the `Styler` object always an attribute `render`? If not, we would run into problems, if we get a `Styler` object with a column named `render` but without the `render` attribute. If a `Styler` object has always an attribute `render`, we could delete the `hasattr` check.

Edit: I changed the check to `not isinstance(df, DataFrame)` to avoid the jinja issues.