BUG: Fix json_normalize throwing AttributeError (#21608)
This PR fixes the bug in `json_normalize` that causes it to throw `AttributeError` when flattening an array of values.
- [ ] closes #21608 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Hello @vuminhle! Thanks for updating the PR.





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

##### Comment last updated on June 23, 2018 at 13:45 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21611?src=pr&el=h1) Report
> Merging [#21611](https://codecov.io/gh/pandas-dev/pandas/pull/21611?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/eb40557a7897a6138b605b1fe5291451027ec01b?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/21611/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/21611?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #21611      +/-   ##
==========================================
+ Coverage    91.9%    91.9%   +<.01%     
==========================================
  Files         153      153              
  Lines       49548    49551       +3     
==========================================
+ Hits        45538    45541       +3     
  Misses       4010     4010
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.3% <100%> (ø)` | :arrow_up: |
| #single | `41.77% <0%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21611?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/json/normalize.py](https://codecov.io/gh/pandas-dev/pandas/pull/21611/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2pzb24vbm9ybWFsaXplLnB5) | `96.96% <100%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21611?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/21611?src=pr&el=footer). Last update [eb40557...7f4e139](https://codecov.io/gh/pandas-dev/pandas/pull/21611?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 closing as stale, if you want to continue working, pls ping and we can re-open. you will need to merge master.