Class B uplink FMAC_CLASSB not set

[terrillmoore]

the class B bit is not set in the uplink message, and therefore the server is still waiting for an uplink message (class A mode).

At line 1890 or so:

    LMIC.frame[OFF_DAT_FCT] = (LMIC.dnConf | LMIC.adrEnabled
                              | (sendAdrAckReq() ? FCT_ADRACKReq : 0)
                              | (end-OFF_DAT_OPTS));

Add here the setting of bit 4 (FCT_CLASSB), if OP_PINGABLE is set. Something like:

    LMIC.frame[OFF_DAT_FCT] = (LMIC.dnConf | LMIC.adrEnabled
                              | (sendAdrAckReq() ? FCT_ADRACKReq : 0)
#if !defined(DISABLE_PING)
                              | (LMIC.opmode & OP_PINGABLE) ? FCT_CLASSB : 0
#endif // ndef DISABLE_PING
                              | (end-OFF_DAT_OPTS));

That worked like a charm. The uplink packet is now marked as class b.

Originally posted by @sualko in #479 (comment)