[DSL] Added correct implementation of Sort#empty?

Closes #317

Author: stephenprater

Diff for: lib/elasticsearch/dsl/search/sort.rb

@@ -44,6 +44,10 @@ def to_hash
           @hash = @value.flatten
           @hash
         end
+
+        def empty?
+          @value.empty? && @args.empty?
+        end
       end
     end
   end

Diff for: test/unit/search_sort_test.rb

@@ -22,6 +22,21 @@ class SearchSortTest < ::Test::Unit::TestCase
           assert_equal( [ { foo: { order: 'desc', mode: 'avg' } } ], subject.to_hash )
         end
 
+        context "when the search is empty" do
+          should "respond with true to empty?" do
+            subject = Elasticsearch::DSL::Search::Sort.new
+            assert_equal subject.empty?, true
+          end
+        end
+
+        context "when search is not empty" do
+          should "respond with false to empty?" do
+            subject = Elasticsearch::DSL::Search::Sort.new foo: { order: 'desc' }
+            assert_equal subject.empty?, false
+          end
+        end
+
+
         should "add fields with the DSL method" do
           subject = Elasticsearch::DSL::Search::Sort.new do
             by :foo