re.c: fixed escaped multibyte char

* re.c (unescape_nonascii): escaped multibyte character should be
  copied as-is, just with checking if the encoding matches.
  https://twitter.com/sakuro/status/972014409986883584

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62718 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_regexp.rb

@@ -515,6 +515,8 @@ class TestRegexp < Test::Unit::TestCase
     s = ".........."
     5.times { s.sub!(".", "") }
     assert_equal(".....", s)
+
+    assert_equal("\\\u{3042}", Regexp.new("\\\u{3042}").source)
   end
 
   def test_equal