CLN: strip whitespaces on `set_td_classes`
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them

A use case with string DataFrames is to construct multiple and add them, needing space separators:

```
cls1 = pd.DataFrame([['', 'cls1 ']])
cls2 = pd.DataFrame([['cls2 ', '']])
cls3 = pd.DataFrame([['cls3 ', 'cls3 ']])
Styler.set_td_classes(cls1 + cls2 + cls3)
```

Stripping cleans up the output for HTML, and might be helpful in other ways..
