make sure capability set is clean before setting new value(s)

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@2685 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/h323/ast_h323.cpp

@@ -858,6 +858,8 @@ int h323_set_capability(int cap, int dtmfMode)
 		return 1;
 	}
 
+	endPoint->SetCapability(0, 0, 0);
+
 	mode = dtmfMode;
 	if (dtmfMode == H323_DTMF_INBAND) {
 	    endPoint->SetSendUserInputMode(H323Connection::SendUserInputAsTone);