Some `package.json` files may contain an `authors` field consisting in
a list of dict. So handle that case to avoid errors such as:
```
[2019-04-11 12:03:21,650: ERROR/ForkPoolWorker-19] Loading failure, updating to `partial` status
Traceback (most recent call last):
File "/usr/lib/python3/dist-packages/swh/loader/core/loader.py", line 893, in load
more_data_to_fetch = self.fetch_data()
File "/usr/lib/python3/dist-packages/swh/loader/npm/loader.py", line 203, in fetch_data
data = next(self.new_versions)
File "/usr/lib/python3/dist-packages/swh/loader/npm/client.py", line 145, in prepare_package_versions
version_data)
File "/usr/lib/python3/dist-packages/swh/loader/npm/client.py", line 200, in _prepare_package_version
author = extract_npm_package_author(package_json)
File "/usr/lib/python3/dist-packages/swh/loader/npm/utils.py", line 92, in extract_npm_package_author
author_data = parse_npm_package_author(package_json['authors'][0])
File "/usr/lib/python3/dist-packages/swh/loader/npm/utils.py", line 52, in parse_npm_package_author
author_str.replace('<>', '').replace('()', ''),
AttributeError: 'dict' object has no attribute 'replace'
```
Related T1629