The contributor method maps to the creator attribute, not contributor

[bmaddy]

Currently, the ContributorsAssociation uses the Dublin Core creator field instead of the contributors field. This becomes confusing for projects that need to keep track of contributors and creators because creators would have to be called something else. I would expect method names to map to dublin core names.

Possibly related to #249

[jcoyne]

https://github.com/projecthydra/curate/blame/1b0822d0c6749b3919f42578948cc91c422d5d29/lib/contributors_association.rb#L91

There's the line. The form element "contributors" is stored as the metadata "creator", these mean different things and we should make them match up. A contributor is not always a creator (Justin is a contributor to Project Hydra, but not the Creator).