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.

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25983?src=pr&el=h1) Report
> Merging [#25983](https://codecov.io/gh/pandas-dev/pandas/pull/25983?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/1172d61aa8cb66e76af2bfdf8a70444802b28c6b?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25983/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25983?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25983      +/-   ##
==========================================
- Coverage   91.85%   91.84%   -0.01%     
==========================================
  Files         175      175              
  Lines       52554    52556       +2     
==========================================
- Hits        48272    48271       -1     
- Misses       4282     4285       +3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.4% <100%> (ø)` | :arrow_up: |
| #single | `41.9% <60%> (-0.08%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25983?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/formats/format.py](https://codecov.io/gh/pandas-dev/pandas/pull/25983/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvZm9ybWF0LnB5) | `97.89% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/25983/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `75% <0%> (-12.5%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/25983/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.79% <0%> (-0.12%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25983/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.71% <0%> (+0.1%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25983?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25983?src=pr&el=footer). Last update [1172d61...71e8b31](https://codecov.io/gh/pandas-dev/pandas/pull/25983?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25983?src=pr&el=h1) Report
> Merging [#25983](https://codecov.io/gh/pandas-dev/pandas/pull/25983?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/35156dc98662eb3633a7784d51709316ad8018d6?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25983/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25983?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25983      +/-   ##
==========================================
- Coverage   91.86%   91.85%   -0.01%     
==========================================
  Files         175      175              
  Lines       52547    52549       +2     
==========================================
- Hits        48271    48268       -3     
- Misses       4276     4281       +5
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.41% <100%> (ø)` | :arrow_up: |
| #single | `41.9% <60%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25983?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/formats/format.py](https://codecov.io/gh/pandas-dev/pandas/pull/25983/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvZm9ybWF0LnB5) | `97.89% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/25983/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `75% <0%> (-12.5%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/25983/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.79% <0%> (-0.12%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25983/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.62% <0%> (-0.11%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25983?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25983?src=pr&el=footer). Last update [35156dc...4ef3149](https://codecov.io/gh/pandas-dev/pandas/pull/25983?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 >  this fully replicates the original issue?

yes. lgtm. ping on green. green. but not all checks ran. needs a restart? hmm yeah, merge master and push again. gbq failure, otherwise good. yeah working on fixing gbq now thanks @simonjayhawkins  I reverted this: https://github.com/pandas-dev/pandas/commit/fc5f292377412fa7bc824d2c2cc69f2ae45bd7f9 as CI is failing