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

Second try
> In src/main/java/de/qabel/core/drop/DropResult.java:
> 
> > @@ -0,0 +1,52 @@
> > +/**
> > + * 
> > + */
> > +package de.qabel.core.drop;
> > +
> > +import java.util.Collections;
> 
> You don't use Collections. But maybe you should.
I use it. I return the unmodifiable list as return value in getPairs