Properties using Optional should have the type: Optional<? extends Type>

[Adrodoc]

For example, the generated builder for the following pojo is not correct:

@GeneratePojoBuilder(withOptionalProperties = Optional.class)
public class Pojo {
  public Number value;
}

The optional-with-method "withValue" should have the following signature:
public PojoBuilder withValue(Optional<? extends Number> value)

but has:
public PojoBuilder withValue(Optional<Number> value)

This is similar to #115

[mkarneim]

Nice!