BUG: Fix index type casting in read_json with orient='table' and float index (#25433)
closes #25433
closes #25435
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25434?src=pr&el=h1) Report
> Merging [#25434](https://codecov.io/gh/pandas-dev/pandas/pull/25434?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/85572de5e7bb188cfecc575ee56786406e79dc79?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `75%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25434/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25434?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25434      +/-   ##
==========================================
- Coverage   91.74%   91.74%   -0.01%     
==========================================
  Files         173      173              
  Lines       52923    52927       +4     
==========================================
+ Hits        48554    48557       +3     
- Misses       4369     4370       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.3% <75%> (-0.01%)` | :arrow_down: |
| #single | `41.73% <75%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25434?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/json/json.py](https://codecov.io/gh/pandas-dev/pandas/pull/25434/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2pzb24vanNvbi5weQ==) | `92.97% <75%> (-0.17%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25434?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/25434?src=pr&el=footer). Last update [85572de...98333da](https://codecov.io/gh/pandas-dev/pandas/pull/25434?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/25434?src=pr&el=h1) Report
> Merging [#25434](https://codecov.io/gh/pandas-dev/pandas/pull/25434?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/85572de5e7bb188cfecc575ee56786406e79dc79?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25434/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25434?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25434      +/-   ##
==========================================
+ Coverage   91.74%   91.74%   +<.01%     
==========================================
  Files         173      173              
  Lines       52923    52928       +5     
==========================================
+ Hits        48554    48559       +5     
  Misses       4369     4369
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.31% <100%> (ø)` | :arrow_up: |
| #single | `41.73% <85.71%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25434?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/json/json.py](https://codecov.io/gh/pandas-dev/pandas/pull/25434/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2pzb24vanNvbi5weQ==) | `93.22% <100%> (+0.08%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25434?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/25434?src=pr&el=footer). Last update [85572de...5c5f3d6](https://codecov.io/gh/pandas-dev/pandas/pull/25434?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jschendel all checks have passed. cc @WillAyd : thought you might want to take a look since you've been more hands-on with the JSON code than me. thanks @albertvillanova keep em coming!