solve issue #107 (TypeError: argument of type 'NewsArticle' is not iterable)
https://github.com/fhamborg/news-please/issues/107
```bash
Traceback (most recent call last):
File "/data/anaconda/envs/giga/lib/python3.6/runpy.py", line 193, in _run_module_as_main
"__main__", mod_spec)
File "/data/anaconda/envs/giga/lib/python3.6/runpy.py", line 85, in _run_code
exec(code, run_globals)
File "/mnt/data/news-please/newsplease/examples/commoncrawl.py", line 169, in <module>
main()
File "/mnt/data/news-please/newsplease/examples/commoncrawl.py", line 165, in main
continue_process=True)
File "/mnt/data/news-please/newsplease/crawler/commoncrawl_crawler.py", line 297, in crawl_from_commoncrawl
log_pathname_fully_extracted_warcs=__log_pathname_fully_extracted_warcs)
File "/mnt/data/news-please/newsplease/crawler/commoncrawl_crawler.py", line 208, in __start_commoncrawl_extractor
log_pathname_fully_extracted_warcs=__log_pathname_fully_extracted_warcs)
File "/mnt/data/news-please/newsplease/crawler/commoncrawl_extractor.py", line 338, in extract_from_commoncrawl
self.__run()
File "/mnt/data/news-please/newsplease/crawler/commoncrawl_extractor.py", line 292, in __run
self.__process_warc_gz_file(local_path_name)
File "/mnt/data/news-please/newsplease/crawler/commoncrawl_extractor.py", line 237, in __process_warc_gz_file
filter_pass, article = self.__filter_record(record)
File "/mnt/data/news-please/newsplease/crawler/commoncrawl_extractor.py", line 121, in __filter_record
publishing_date = self.__get_publishing_date(warc_record, article)
File "/mnt/data/news-please/newsplease/crawler/commoncrawl_extractor.py", line 140, in __get_publishing_date
if 'publish_date' in article:
TypeError: argument of type 'NewsArticle' is not iterable
```
