BUG: Allow merging on Index vectors
This behavior used to work in v0.19.0 and
is consistent with the documentation.

Closes #19038

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19073?src=pr&el=h1) Report
> Merging [#19073](https://codecov.io/gh/pandas-dev/pandas/pull/19073?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/c0e37670dd578728f988a33bc21bc173b336249a?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/19073/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/19073?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19073      +/-   ##
==========================================
+ Coverage   91.53%   91.53%   +<.01%     
==========================================
  Files         148      148              
  Lines       48688    48690       +2     
==========================================
+ Hits        44566    44568       +2     
  Misses       4122     4122
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.9% <100%> (ø)` | :arrow_up: |
| #single | `41.63% <40%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19073?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/dtypes/api.py](https://codecov.io/gh/pandas-dev/pandas/pull/19073/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2FwaS5weQ==) | `100% <ø> (ø)` | :arrow_up: |
| [pandas/core/reshape/merge.py](https://codecov.io/gh/pandas-dev/pandas/pull/19073/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9tZXJnZS5weQ==) | `94.21% <100%> (ø)` | :arrow_up: |
| [pandas/core/dtypes/inference.py](https://codecov.io/gh/pandas-dev/pandas/pull/19073/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2luZmVyZW5jZS5weQ==) | `98.41% <100%> (+0.05%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19073?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/19073?src=pr&el=footer). Last update [c0e3767...1a553bc](https://codecov.io/gh/pandas-dev/pandas/pull/19073?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jreback : Comments have been addressed, and all is green.  PTAL. very nice @gfyoung thanks!