Set "modified:" from "date:" when the latter was set from the filesystem (#2748)

Sets the `modified:` metadata from `date:` if the user asked us
to use the filesystem for determining timestamps. Fixes #2497

* Added tests (ab)using the HTML reader a bit

There seems to be no way to test this generically since we need
to produce a valid document with meta information to be able to
compare, hence I used the lightest reader out there to do the
parsing of the samples.

* Fixed tests for generators since there were 4 more articles
introduced to the test content directory.

pelican/readers.py

@@ -685,6 +685,7 @@ def path_metadata(full_path, source_path, settings=None):
         if settings.get('DEFAULT_DATE', None) == 'fs':
             metadata['date'] = datetime.datetime.fromtimestamp(
                 os.stat(full_path).st_mtime)
+            metadata['modified'] = metadata['date']
 
         # Apply EXTRA_PATH_METADATA for the source path and the paths of any
         # parent directories. Sorting EPM first ensures that the most specific