Make `order` option Rails 4.0.1-compatible

The way that we figure out whether the value which is passed to `order`
is the same as the `order` that the association was defined with is by
creating an ActiveRecord::Relation object from the given `order` and
comparing it with the Relation stored under the association.
Specifically, every time you call `order` on a Relation, it appends the
value you've given to an internal `order_values` array, so we actually
access this array and compare the two between the two Relation objects.

Currently we assume that this `order_values` array is an array of
strings, so it's very easy to compare. That was valid pre-Rails 4.0.1,
but it looks like as per [these][1] [commits][2], it's now possible for
`order_values` to be an array of Arel nodes. So, to make a proper
comparison, we have to convert any Arel nodes to strings by calling
 #to_sql on them.

[1]: https://github.com/rails/rails/commit/d345ed4
[2]: f83c9b10b4

lib/shoulda/matchers/active_record/association_matchers/model_reflector.rb

@@ -37,15 +37,26 @@ module Shoulda # :nodoc:
 
           def extract_relation_clause_from(relation, name)
             case name
-              when :conditions then relation.where_values_hash
-              when :order      then relation.order_values.join(', ')
-              else                  raise ArgumentError, "Unknown clause '#{name}'"
+            when :conditions
+              relation.where_values_hash
+            when :order
+              relation.order_values.map { |value| value_as_sql(value) }.join(', ')
+            else
+              raise ArgumentError, "Unknown clause '#{name}'"
             end
           end
 
           private
 
           attr_reader :subject, :name
+
+          def value_as_sql(value)
+            if value.respond_to?(:to_sql)
+              value.to_sql
+            else
+              value
+            end
+          end
         end
       end
     end