BUG: Fix render as column name in to_excel

[phofl]

• closes BUG: writing dataframe to excel if one of the rows has name "render" fails #34331
• tests added / passed
• passes black pandas
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• 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.

[dsaxton]

@phofl You can add a bug fix whatsnew note under the I/O section in pandas/doc/source/whatsnew/v1.1.0.rst

[phofl]

@phofl You can add a bug fix whatsnew note under the I/O section in pandas/doc/source/whatsnew/v1.1.0.rst

Àdded the whats new entry

[jreback]

thanks @phofl very nice!