Created by: gracewashere
Previously, Administrate would try to look up the incorrect dashboard
class for has_many :through relationships with a source option
because it wasn't taking that option into account.
Also, include tests to make sure we don't set the class_name option unless it's needed
The class_name option was being set incorrectly for has_many :through
without a :source option, and for has_many :through -> has_many
nested relationships