Field order issue in loading json
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Hi @luyug, thanks for reporting.

The good news is that we fixed this issue only 9 days ago: #2507.

The patch is already in the master branch of our repository and it will be included in our next `datasets` release version 1.9.0.

Feel free to reopen the issue if the problem persists.
## Describe the bug
The `load_dataset` function expects columns in alphabetical order when loading json files.

Similar bug was previously reported for csv in #623 and fixed in #684.
## Steps to reproduce the bug

For a json file `j.json`,
```
{"c":321, "a": 1, "b": 2}
```
Running the following,
```
f= datasets.Features({'a': Value('int32'), 'b': Value('int32'), 'c': Value('int32')})
json_data = datasets.load_dataset('json', data_files='j.json', features=f)
```


## Expected results
A successful load.
## Actual results
```
File "pyarrow/table.pxi", line 1409, in pyarrow.lib.Table.cast
ValueError: Target schema's field names are not matching the table's field names: ['c', 'a', 'b'], ['a', 'b', 'c']
```

## Environment info
- `datasets` version: 1.8.0
- Platform: Linux-3.10.0-957.1.3.el7.x86_64-x86_64-with-glibc2.10
- Python version: 3.8.8
- PyArrow version: 3.0.0


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