Parse error with required kwargs and omitted parens

Given a method definition like this:

``` ruby
def foo arg:
  123
end
```

`mruby` 3.3.0 parses the definition as:

``` ruby
def foo(arg: 123)
end
```

… while C Ruby parses it as:

``` ruby
def foo(arg:)
  123
end
```

I could find no documentation that suggests this is a deliberate change in behavior.
