BUG: to_numeric downcast = 'unsigned' would not un-sign a 0 value.
- [ ] closes #14401
- [ ] tests added / passed
- [ ] passes `git diff upstream/master | flake8 --diff`
- [ ] whatsnew entry

Have to figure out whats going wrong with the build still. Will let you know as soon as I do.

can you rebase this
 ## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14504?src=pr) is 85.28% (diff: 100%)

> Merging [#14504](https://codecov.io/gh/pandas-dev/pandas/pull/14504?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will not change coverage

``` diff
@@             master     #14504   diff @@
==========================================
  Files           140        140          
  Lines         50693      50693          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          43235      43235          
  Misses         7458       7458          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [726efc7...07264cd](https://codecov.io/gh/pandas-dev/pandas/compare/726efc7cd55744eb88636dd6f11293f18355a10a...07264cd9bfb3b49b8a1b9769991a155c7b92cdde?src=pr)
 @verhalenn There is still a test failing. Do you have time to look into this?
 @jorisvandenbossche I have been but I'm having trouble replicating it.
 @verhalenn Apparently this is a bug in older numpy versions:

```
>>> np.__version__
'1.8.2'
>>> arr = np.array([-128, 127])
>>> arr2 = arr.astype('int8')
>>> np.allclose(arr, arr2)
False
```

```
>>> np.__version__
'1.11.2'
>>> arr = np.array([-128, 127])
>>> arr2 = arr.astype('int8')
>>> np.allclose(arr, arr2)
True
```

So you can skip the failing test for numpy versions of 1.8 or lower.
 That should do. If there is anything you would like different, let me know.

Also thanks @jorisvandenbossche for the help.
 @verhalenn See here for an example how to skip a test for a numpy version: https://github.com/pandas-dev/pandas/blob/52f31d470d779204e4c1388cdb56351c68332c3f/pandas/tests/series/test_quantile.py#L76

Can you also move the whatsnew note to the 0.19.2 file ?
 thanks!
