Fix a bug in `Marshal.load` that caused it to loop indefinitely when
trying to autoload a constant that resolved to a different name.

This could occur when marshalling an ActiveRecord 4.0 object (e.g. into
memcached) and then trying to unmarshal it with Rails 4.2. The
marshalled payload contains a reference to
`ActiveRecord::ConnectionAdapters::Mysql2Adapter::Column`, which in
Rails 4.2 resolves to
`ActiveRecord::ConnectionAdapters::AbstractMysqlAdapter::Column`.