ruby2_keywords warnings: Quote non-UTF8 method names fully

It used to quote only part of the method name because NUL byte in
the method terminates the C string:

```
(irb)> "abcdef".encode("UTF-16LE").bytes
=> [97, 0, 98, 0, 99, 0, 100, 0, 101, 0, 102, 0]
```

```
expected: /abcdef/
actual: warning: Skipping set of ruby2_keywords flag for a (method not defined in Ruby)\n".
```

1 files changed, 6 insertions, 0 deletions

diff --git a/test/ruby/test_keyword.rb b/test/ruby/test_keyword.rb
index 6da1b73c15..4563308fa2 100644
--- a/test/ruby/test_keyword.rb
+++ b/test/ruby/test_keyword.rb
@@ -2794,6 +2794,12 @@ class TestKeywordArguments < Test::Unit::TestCase
       assert_nil(c.send(:ruby2_keywords, :bar))
     end
 
+    utf16_sym = "abcdef".encode("UTF-16LE").to_sym
+    c.send(:define_method, utf16_sym, c.instance_method(:itself))
+    assert_warn(/abcdef/) do
+      assert_nil(c.send(:ruby2_keywords, utf16_sym))
+    end
+
     o = Object.new
     class << o
       alias bar p