BUG: startswith function gives misleading results when passed a series #3485

- [x] closes #3485
- [] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [ ] whatsnew entry
``` python import pandas
d = pandas.DataFrame([['hi,hi 2'],['bye','bye 2']], columns=['one','two'])
print(d.one.str.startswith(d.two))
```