Omitted default argument values

Under some conditions, for instance if a type parameter is present, the default argument value of a method parameter is not printed. Try for example:

```scala
trait Test {
  def f   (a: Seq[List[Any]] = Seq.empty[List[Any]])
  def g[A](a: Seq[List[A]]   = Seq.empty[List[A]])
}
```

Imo it would be satisfactory to print the textual representation of the default argument (see also #3721).