Fix #111 Drop::sendAndForget can return a false value

Second try
> In src/main/java/de/qabel/core/drop/Drop.java:
> 
> > @@ -181,7 +199,7 @@ private String serialize(DropMessage<T> message) {
> >       * @param plainJson plain Json String
> >       * @return deserialized Dropmessage
> >       */
> > -    private DropMessage deserialize(String plainJson) {
> > +    private DropMessage<T> deserialize(String plainJson) {
> 
> What has this change to do with this PR and - more importantly - why
> did you change that?
As I know there was an idea from @aBothe to set the DropMessage<T> from
every send function to <? extends DropMessage>. So I add it. But this
patch I do not add. Why this already exists I do not know.

I remove that