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 @@ module ActiveRecord
       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 @@ module ActiveRecord
     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 @@ module ActiveRecord
 
       case scope
       when :order
-        self.reverse_order_value = false
         result = []
       else
         result = [] unless single_val_method
@@ -1031,7 +1032,6 @@ module ActiveRecord
     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 @@ module ActiveRecord
     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 !relation.reverse_order_value
+
+      assert_equal 'title DESC', relation.order_values.first
+      assert_equal 'comments_count ASC', relation.order_values.last
+
+
+      relation.reverse_order!
+
+      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 @@ class RelationTest < ActiveRecord::TestCase
     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