Unary method call doesn't drop the trailing `@`

For comparision:

```sh
$ ruby-parse --33 -e ":~@"
(sym :~)

$ ruby-parse --33 -e "foo.~@"
(send
  (send nil :foo) :"~@")
```

The same rules exist here, ruby will actually call the `~` method. Prism gets this wrong right now as well, just opening this to reference the added translation test over there.