ModelMethods#apply_depositor_metadata should accept a user or a string
like the method it overrides in HydraHead.  See:
https://github.com/projecthydra/hydra-head/blob/9a5b2728be2046125d09376a6d78ad06d26548c3/hydra-core/lib/hydra/model_methods.rb#L12
Merge pull request #173 from projecthydra/apply_depositor_object

ModelMethods#apply_depositor_metadata should accept a user or a string