Fix code hover for methods with recursive generics

Consider the following:
```groovy
class Pogo {
  Date date
}
def cmp = Comparator.<Pogo,Date>comparing{ it.date }
```

Hovering over "comparing" fails to display javadoc due to error in binding key for `static <T, U extends Comparable<? super U>> Comparator<T> comparing(Function<? super T, ? extends U>)`.

See #920