Endless method with modifier method does not parse

Endless methods definitions preceded by some modifier method, like `private`, does not currently parse (legacy parser):

```
$ jruby -e 'private def foo = puts "Hello"'
SyntaxError: -e:1: syntax error, unexpected string literal
private def foo = puts "Hello"
                       ^
```

This causes a failure in CRuby's test_syntax.rb @ `test_methoddef_endless_command`.