named_captures: fix incompatibility with MatchData#named_captures

Fix GH-145

`MatchData#named_captures` use the last matched value for each name.

Reported by Linus Sellberg. Thanks!!!

Author: kou

ext/strscan/strscan.c

@@ -2211,7 +2211,10 @@ named_captures_iter(const OnigUChar *name,
     VALUE value = RUBY_Qnil;
     int i;
     for (i = 0; i < back_num; i++) {
-        value = strscan_aref(data->self, INT2NUM(back_refs[i]));
+        VALUE v = strscan_aref(data->self, INT2NUM(back_refs[i]));
+        if (!RB_NIL_P(v)) {
+            value = v;
+        }
     }
     rb_hash_aset(data->captures, key, value);
     return 0;

test/strscan/test_stringscanner.rb

@@ -967,6 +967,12 @@ def test_named_captures
     assert_equal({}, scan.named_captures)
   end
 
+  def test_named_captures_same_name_union
+    scan = StringScanner.new("123")
+    assert_equal(1, scan.match?(/(?<number>0)|(?<number>1)|(?<number>2)/))
+    assert_equal({"number" => "1"}, scan.named_captures)
+  end
+
   def test_scan_integer
     s = create_string_scanner('abc')
     assert_equal(3, s.match?(/(?<a>abc)/)) # set named_captures