Issue: load_dataset for CSV files not working

Comment:
> Similar to #622, I've noticed there is a problem when trying to load a CSV file with datasets.
> 
> `from datasets import load_dataset`
> `dataset = load_dataset("csv", data_files=["./sample_data.csv"], delimiter="\t", column_names=["title", "text"], script_version="master")`
> 
> Displayed error:
> `... ArrowInvalid: CSV parse error: Expected 2 columns, got 1`

I'm also facing the same issue when trying to load from a csv file locally:

```python
from nlp import load_dataset
dataset = load_dataset('csv', data_files='sample_data.csv')
```

Error when executed from Google Colab:
```python
ArrowInvalid                              Traceback (most recent call last)
<ipython-input-34-79a8d4f65ed6> in <module>()
      1 from nlp import load_dataset
----> 2 dataset = load_dataset('csv', data_files='sample_data.csv')

9 frames
/usr/local/lib/python3.7/dist-packages/nlp/load.py in load_dataset(path, name, version, data_dir, data_files, split, cache_dir, features, download_config, download_mode, ignore_verifications, save_infos, **config_kwargs)
    547     # Download and prepare data
    548     builder_instance.download_and_prepare(
--> 549         download_config=download_config, download_mode=download_mode, ignore_verifications=ignore_verifications,
    550     )
    551 

/usr/local/lib/python3.7/dist-packages/nlp/builder.py in download_and_prepare(self, download_config, download_mode, ignore_verifications, try_from_hf_gcs, dl_manager, **download_and_prepare_kwargs)
    461                 if not downloaded_from_gcs:
    462                     self._download_and_prepare(
--> 463                         dl_manager=dl_manager, verify_infos=verify_infos, **download_and_prepare_kwargs
    464                     )
    465                 # Sync info

/usr/local/lib/python3.7/dist-packages/nlp/builder.py in _download_and_prepare(self, dl_manager, verify_infos, **prepare_split_kwargs)
    535             try:
    536                 # Prepare split will record examples associated to the split
--> 537                 self._prepare_split(split_generator, **prepare_split_kwargs)
    538             except OSError:
    539                 raise OSError("Cannot find data file. " + (self.manual_download_instructions or ""))

/usr/local/lib/python3.7/dist-packages/nlp/builder.py in _prepare_split(self, split_generator)
    863 
    864         generator = self._generate_tables(**split_generator.gen_kwargs)
--> 865         for key, table in utils.tqdm(generator, unit=" tables", leave=False):
    866             writer.write_table(table)
    867         num_examples, num_bytes = writer.finalize()

/usr/local/lib/python3.7/dist-packages/tqdm/notebook.py in __iter__(self, *args, **kwargs)
    213     def __iter__(self, *args, **kwargs):
    214         try:
--> 215             for obj in super(tqdm_notebook, self).__iter__(*args, **kwargs):
    216                 # return super(tqdm...) will not catch exception
    217                 yield obj

/usr/local/lib/python3.7/dist-packages/tqdm/std.py in __iter__(self)
   1102                 fp_write=getattr(self.fp, 'write', sys.stderr.write))
   1103 
-> 1104         for obj in iterable:
   1105             yield obj
   1106             # Update and possibly print the progressbar.

/usr/local/lib/python3.7/dist-packages/nlp/datasets/csv/ede98314803c971fef04bcee45d660c62f3332e8a74491e0b876106f3d99bd9b/csv.py in _generate_tables(self, files)
     78                 read_options=self.config.pa_read_options,
     79                 parse_options=self.config.pa_parse_options,
---> 80                 convert_options=self.config.convert_options,
     81             )
     82             yield i, pa_table

/usr/local/lib/python3.7/dist-packages/pyarrow/_csv.pyx in pyarrow._csv.read_csv()

/usr/local/lib/python3.7/dist-packages/pyarrow/error.pxi in pyarrow.lib.pyarrow_internal_check_status()

/usr/local/lib/python3.7/dist-packages/pyarrow/error.pxi in pyarrow.lib.check_status()

ArrowInvalid: CSV parse error: Expected 1 columns, got 8
```

Version:
```
nlp==0.4.0
```