Send commissioner set after petitioning (#1486)

If AddJoiner (or other joiner list methods) are called before the
commissioner manages to finish the petitioning, then network steering
data is not properly updated and joiners don't join the network.

src/core/meshcop/commissioner.cpp

@@ -633,6 +633,8 @@ void Commissioner::HandleLeaderPetitionResponse(Coap::Header *aHeader, Message *
     mTransmitAttempts = 0;
     mTimer.Start(Timer::SecToMsec(kKeepAliveTimeout) / 2);
 
+    SendCommissionerSet();
+
 exit:
 
     if (retransmit)