Fix issue with adding editors to a generic work

Adding editors to a generic work suffered from the same problem
as https://github.com/projecthydra-labs/curate/pull/526.  So we
implemented the same fix for the prefix (using "generic_work"
instead of "genericwork")

HYDRASIR-529 #close
Merge pull request #540 from projecthydra-labs/feature/HYDRASIR-529

Fix issue with adding editors to a generic work