When building the join logic, do not use include the first link

* This adds an unecessary join to the statement, and can also cause some
  aliasing issues in self referrential joins.  The aliasing problem will
  still need to be resolved for very complex joins, but this makes the
  joins for m:m associations more efficient in the meantime.

lib/dm-core/adapters/data_objects_adapter.rb

@@ -439,7 +439,7 @@ def join_statement(query, qualify)
 
           statement = ''
 
-          query.links.reverse_each do |relationship|
+          query.links[0..-1].reverse_each do |relationship|
             model = if relationship.source_model == previous_model
               relationship.target_model
             else