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.
