Subexp calls work fine with local flags, add spec

closes #27

Author: jaynetics

spec/lib/js_regex/converter/backreference_converter_spec.rb

@@ -272,6 +272,13 @@
         .and keep_matching('ababb', 'acabb')
         .and keep_not_matching('ababc', 'acabc')
     end
+
+    it 'uses the correct local flags / options (those that apply to the target)' do
+      expect(/(a)(?i)\g<1>/).to\
+      become(/(a)(a)/)
+        .and keep_matching('aa')
+        .and keep_not_matching('aA')
+    end
   end
 
   context 'when handling multiplexed named groups' do