Fix issue #505: DIET: doctype is actually ignored

Fix issue #505

Again, not sure if it is the best approach.

Btw, I didn't know if I should use:

```d
case "!!!":
    ln = ln[j + 3 .. $]; 
    goto case "doctype";
case "doctype"
...
```
Or the one I used in code.

Also I am not sure if code guidelines are correct.
Btw what is maximum column line, 120?