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".
```