BUG: na_rep given precedence in to_html
- [x] closes #13828
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

~~I know new tests are appreciated from the get-go, but it's the end of the day for me. I'll add new tests and the appropriate whatsnew entry tomorrow.~~

## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/13911?src=pr) is 84.64% (diff: 90.00%)
> Merging [#13911](https://codecov.io/gh/pandas-dev/pandas/pull/13911?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **<.01%**

```diff
@@             master     #13911   diff @@
==========================================
  Files           144        144          
  Lines         51016      51024     +8   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43184      43191     +7   
- Misses         7832       7833     +1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [3ccb501...685c6d2](https://codecov.io/gh/pandas-dev/pandas/compare/3ccb50131b698bccee21780c660a70bed87396d0...685c6d2f786c6851c4d59c07db7cfc10e63f6e2f?src=pr) @jreback I think I understood what you meant by iterating over non-null values. Please have a look.
 @jreback
 can you rebase / update?
 Rebased and all green.
 lgtm. @jorisvandenbossche 
 i think this got lost in the shuffle, can you update. Rebased, moved whatsnew to 0.20.0, need to look into the code duplication @jreback mentioned. can you rebase / update closing as stale. please ping if you'd like to continue.