to_html formatter not called for float values in a mixed-type column
- [x] closes #13021
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

can't shortcut completely without changing existing tested behaviour in a couple of tests.

for instance, the expected output for `test_to_string_with_formatters` in `pandas\tests\io\formats\test_format.py` has two spaces between float and object column, whereas shortcuting would only have one (the same as between the int and float columns)

```
  int  float    object
0 0x1 [ 1.0]  -(1, 2)-
1 0x2 [ 2.0]    -True-
2 0x3 [ 3.0]   -False-
```
if it's OK to change a couple of tests, we can remove this subtle leading_space bug in this PR.
