Fix bean getter/setter type mismatch

Mismatch was causing failures on Travis CI because the getter and setter types did not match:

    Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'noUserSelectionAction' defined in URL [file:/opt/dryad-test//config/workflow-actions.xml]: Error setting property values; nested exception is org.springframework.beans.NotWritablePropertyException: Invalid property 'processingAction' of bean class [org.dspace.workflow.actions.UserSelectionActionConfig]: Bean property 'processingAction' is not writable or has an invalid setter method. Does the parameter type of the setter match the return type of the getter?"