Fixed the case of multiple publishers and contributors for DublinCore metadata

[ahmdthr]

Overview

Fixes issue#1069
When requesting to download DublinCore metedata, the request fails if there are any publishers because the publishers are in a list form. This PR handles the list of publishers and contributors.

Related Issue / Discussion

issue#1069

Additional Information

Contributions and Licensing

(as per https://github.com/geopython/pycsw/blob/master/CONTRIBUTING.rst#contributions-and-licensing)

• I'd like to contribute [feature X|bugfix Y|docs|something else] to pycsw. I confirm that my contributions to pycsw will be compatible with the pycsw license guidelines at the time of contribution.
• I have already previously agreed to the pycsw Contributions and Licensing Guidelines