Add fallback for dct:issued and dct:modified in resources

[seitenbau-govdata]

We'd like to add a fallback for the serialization of the resource dates dct:issued and dct:modified, in the same way it is implemented for the dataset. To do so we use the CKAN internal dates created and _metadatamodified of the resource.

[amercader]

This makes sense @seitenbau-govdata , can you merge the latest master branch? that should fix the lint testing issues

[seitenbau-govdata]

@amercader Done. The pull request is now based on the current master. Thanks for fixing the testing issues.