Revert back to join rather than include.

AR2's `join` will make the found model instance read-only, hence the
previous change to use `include`. However, `include` will add columns
from the slug table to the query, so queries that use unqualified column
names and work without FriendlyId, will fail when FriendlyId is used if
a column name from the slug table is used in a scope or find conditions.

This change simply sets the `:readonly` option to `false` unless it is
explicitly set to something truthy. This doesn't feel entirely right
but in practice ends up working better.

closes #75

lib/friendly_id/active_record_adapter/finders.rb

@@ -36,11 +36,12 @@ def find_some
           parse_ids!
           scope = some_friendly_scope
           if use_slugs? && @friendly_ids.present?
-            scope = scope.scoped(:include => :slugs)
+            scope = scope.scoped(:joins => :slugs)
             if fc.scope?
               scope = scope.scoped(:conditions => {:slugs => {:scope => scope_val}})
             end
           end
+          options[:readonly] = false unless options[:readonly]
           @result = scope.all(options).uniq
           validate_expected_size!
           @result.each { |record| record.friendly_id_status.name = id }
@@ -62,8 +63,9 @@ def find_one_using_cached_slug
 
         def find_one_using_slug
           name, seq = id.to_s.parse_friendly_id
-          scope = scoped(:include => :slugs, :conditions => {:slugs => {:name => name, :sequence => seq}})
+          scope = scoped(:joins => :slugs, :conditions => {:slugs => {:name => name, :sequence => seq}})
           scope = scope.scoped(:conditions => {:slugs => {:scope => scope_val}}) if fc.scope?
+          options[:readonly] = false unless options[:readonly]
           @result = scope.first(options)
           assign_status
         end