Build the reverse_order on its proper method.

The reverse_order method was using a flag to control if the order should
be reversed or not. Instead of using this variable just build the reverse order
inside its proper method.

This implementation was leading to an unexpected behavior when using
reverse_order and then applying reorder(nil).

Example:
  Before
    Post.order(:name).reverse_order.reorder(nil)
    # => SELECT "posts".* FROM "posts"   ORDER BY "posts"."id" DESC

  After
    Post.order(:name).reverse_order.reorder(nil)
   # => SELECT "posts".* FROM "posts"

activerecord/lib/active_record/relation/merger.rb

@@ -139,7 +139,6 @@ def merge_multi_values
       def merge_single_values
         relation.from_value          = values[:from] unless relation.from_value
         relation.lock_value          = values[:lock] unless relation.lock_value
-        relation.reverse_order_value = values[:reverse_order]
 
         unless values[:create_with].blank?
           relation.create_with_value = (relation.create_with_value || {}).merge(values[:create_with])

activerecord/lib/active_record/relation/query_methods.rb

@@ -825,7 +825,9 @@ def reverse_order
     end
 
     def reverse_order! # :nodoc:
-      self.reverse_order_value = !reverse_order_value
+      orders = order_values.uniq
+      orders.reject!(&:blank?)
+      self.order_values = reverse_sql_order(orders)
       self
     end
 
@@ -871,7 +873,6 @@ def symbol_unscoping(scope)
 
       case scope
       when :order
-        self.reverse_order_value = false
         result = []
       else
         result = [] unless single_val_method
@@ -1031,7 +1032,6 @@ def array_of_strings?(o)
     def build_order(arel)
       orders = order_values.uniq
       orders.reject!(&:blank?)
-      orders = reverse_sql_order(orders) if reverse_order_value
 
       arel.order(*orders) unless orders.empty?
     end

activerecord/test/cases/relation/mutation_test.rb

@@ -107,10 +107,18 @@ def relation
     end
 
     test 'reverse_order!' do
-      assert relation.reverse_order!.equal?(relation)
-      assert relation.reverse_order_value
+      relation = Post.order('title ASC, comments_count DESC')
+
+      relation.reverse_order!
+
+      assert_equal 'title DESC', relation.order_values.first
+      assert_equal 'comments_count ASC', relation.order_values.last
+
+
       relation.reverse_order!
-      assert !relation.reverse_order_value
+
+      assert_equal 'title ASC', relation.order_values.first
+      assert_equal 'comments_count DESC', relation.order_values.last
     end
 
     test 'create_with!' do

activerecord/test/cases/relations_test.rb

@@ -1424,6 +1424,18 @@ def test_automatically_added_reorder_references
     assert_equal [], scope.references_values
   end
 
+  def test_order_with_reorder_nil_removes_the_order
+    relation = Post.order(:title).reorder(nil)
+
+    assert_nil relation.order_values.first
+  end
+
+  def test_reverse_order_with_reorder_nil_removes_the_order
+    relation = Post.order(:title).reverse_order.reorder(nil)
+
+    assert_nil relation.order_values.first
+  end
+
   def test_presence
     topics = Topic.all