Fix issuables sort direction button parameters

Fix labels parameter being deleted from filter
when clicking sort direction button on issues/merge requests search bar.

app/helpers/sorting_helper.rb

@@ -164,7 +164,7 @@ module SortingHelper
     reverse_sort = issuable_reverse_sort_order_hash[sort_value]
 
     if reverse_sort
-      reverse_url = page_filter_path(sort: reverse_sort)
+      reverse_url = page_filter_path(sort: reverse_sort, label: true)
     else
       reverse_url = '#'
       link_class += ' disabled'

spec/helpers/sorting_helper_spec.rb

@@ -21,7 +21,11 @@ describe SortingHelper do
 
   describe '#issuable_sort_direction_button' do
     before do
-      allow(self).to receive(:request).and_return(double(path: 'http://test.com', query_parameters: {}))
+      allow(self).to receive(:request).and_return(double(path: 'http://test.com', query_parameters: { label_name: 'test_label' }))
+    end
+
+    it 'keeps label filter param' do
+      expect(issuable_sort_direction_button('created_date')).to include('label_name=test_label')
     end
 
     it 'returns icon with sort-highest when sort is created_date' do