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

Second try
> In src/main/java/de/qabel/core/drop/Drop.java:
> 
> > -			} catch (InvalidKeyException e) {
> > -				logger.error("Invalid key in contact. Cannot send message!");
> > -			}           
> > +    public DropResult sendAndForget(DropMessage<T> message,
> > +    		Collection<Contact> contacts) {
> > +    	DropResult result;
> > +    	boolean ok = true;;
> > +    	List<DropResultPair> pairs;
> > +    	
> > +    	pairs = new ArrayList<DropResultPair>();
> > +        
> > +        for (Contact contact : contacts) {
> > +        	DropResultPair pair;
> > +        	
> > +        	pair = new DropResultPair(contact, this.sendAndForget(message, contact));
> > +        	if (pair.isOk() == false) {
> 
> a simple ok = pair.isOk() would have sufficed.
No. That is not true because ok says minimum one drop server of each
contacts received the message. If there are three contacts and the
second one does not received the message and the third will reach it the
function returns true but that is not the truth. That was also the
problem in the origin code.

Here is the rest of the code:

        		ok = pair.isOk();
        	}
        	pairs.add(pair);
        }
        result = new DropResult(ok, pairs);

        return (result);