Add a MatchData#named_captures spec that exercises capture name reuse with optional patterns.

Author: nirvdrum

spec/ruby/core/matchdata/named_captures_spec.rb

@@ -9,5 +9,9 @@
     it 'prefers later captures' do
       /\A(?<a>.)(?<b>.)(?<b>.)(?<a>.)\z/.match('0123').named_captures.should == { 'a' => '3', 'b' => '2' }
     end
+
+    it 'returns the latest matched capture, even if a later one that does not match exists' do
+      /\A(?<a>.)(?<b>.)(?<b>.)(?<a>.)?\z/.match('012').named_captures.should == { 'a' => '0', 'b' => '2' }
+    end
   end
 end