BUG: excel index label can be an array
Fixes "ValueError: The truth value of an array with more than one element is ambiguous." where self.index_label could be a list, tuple np.ndarray, or Index type

closes #20622 
Hello @saxaholic! Thanks for updating the PR.



Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on April 06, 2018 at 15:18 Hours UTC Could you add tests (that fail without the change) and a release note? http://pandas.pydata.org/pandas-docs/stable/contributing.html I will try if I have time. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20623?src=pr&el=h1) Report
> Merging [#20623](https://codecov.io/gh/pandas-dev/pandas/pull/20623?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/6d610a4d9393c0d0335267dc3252ccabb9e51e43?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20623/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/20623?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20623      +/-   ##
==========================================
+ Coverage   91.82%   91.84%   +0.02%     
==========================================
  Files         153      153              
  Lines       49256    49256              
==========================================
+ Hits        45227    45241      +14     
+ Misses       4029     4015      -14
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.23% <100%> (+0.02%)` | :arrow_up: |
| #single | `41.9% <0%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20623?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/formats/excel.py](https://codecov.io/gh/pandas-dev/pandas/pull/20623/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvZXhjZWwucHk=) | `97.36% <100%> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/20623/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `84.73% <0%> (+0.2%)` | :arrow_up: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/20623/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `66.81% <0%> (+1.73%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20623?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/20623?src=pr&el=footer). Last update [6d610a4...b5752b7](https://codecov.io/gh/pandas-dev/pandas/pull/20623?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 closing as stale