headers_make_unique output not always unique

Ran into a situation when a string list is not converted into one with unique items:
```
$ python --version
Python 3.9.5

$ pip freeze | grep messytables
messytables==0.15.2

$ python -c "import messytables; print(messytables.headers.headers_make_unique([ 'a', 'a', 'a_2' ]))"
['a_1', 'a_2', 'a_2']
```