Fixes chained join method's signature in the QueryBuilder

[derenio]

No description provided.

[jeffdgr8]

@greenrobot @greenrobot-team can this fix be looked at merging? Anyone that's doing chained joins has to be casting the sourceJoin argument here to the incorrect Join<?, T> in order for the method to accept the argument. Your own chained join sample won't compile with the code as is.

Part of the problem is that the return type of the method actually isn't Join<T, J>, but rather Join<J, K>, as the QueryBuilder's T is not the intended type of the chained join, rather it's joining to a new K type, from the J type of the previous join. The method signature would be more appropriate as:

public <J, K> Join<J, K> join(Join<?, J> sourceJoin, Property sourceProperty, Class<K> destinationEntityClass, Property destinationProperty)

And addJoin() would more correctly be:

private <J> Join<?, J> addJoin(String sourceTablePrefix, Property sourceProperty, AbstractDao<J, ?> destinationDao, Property destinationProperty)

as the join may be from T in the non-chained join() methods or it may be from a chained type from the chained join() method.

[jeffdgr8]

I created a pull request with the fix I described above: #913.

This pull request fix will work, but the type of the Join generics are still incorrect, which although they're not actually used, can be misleading.