Merge pull request #49 from hbz/nwbib-issue-57-medium

Support `medium=` query parameter for lobid-resources
Tweak XML response for `format=full` requests

- Set content type to text/xml for single response
- Don't wrap multiple results into surrounding element
Merge pull request #50 from hbz/xml-response

Tweak XML response for `format=full` requests
Fix caching issue for faceted search

See https://github.com/hbz/nwbib/issues/57