Multiple self-referencing eager loads don't join properly

This is from #4463 JoinAssociation#aliased_table_name_for bug affecting AR find_with_associations? -- decided it's probably better to create a new ticket than submit a fix to an already committed ticket. I made a small, silly error in the previously committed patch, checking is_a? instead of class equality. Since JoinAssociation inherits from JoinBase, this can cause a stashed association join to be grafted onto the JoinBase instead of a JoinAssociation in the edge case of multiple-self-joining eager loads. Patch with test included. Sorry about that!