Adafruit_MQTT (API): the payload of publish method is read-only

The payload param provided in publish is a const. This change
fixes the API to ensure caller that this is the case for all
variations of the publish method.

Adafruit_MQTT.cpp

@@ -311,10 +311,10 @@ bool Adafruit_MQTT::disconnect() {
 }
 
 bool Adafruit_MQTT::publish(const char *topic, const char *data, uint8_t qos) {
-  return publish(topic, (uint8_t *)(data), strlen(data), qos);
+  return publish(topic, (const uint8_t *)(data), strlen(data), qos);
 }
 
-bool Adafruit_MQTT::publish(const char *topic, uint8_t *data, uint16_t bLen,
+bool Adafruit_MQTT::publish(const char *topic, const uint8_t *data, uint16_t bLen,
                             uint8_t qos) {
   // Construct and send publish packet.
   uint16_t len =
@@ -680,8 +680,8 @@ uint16_t Adafruit_MQTT::packetAdditionalLen(uint16_t currLen) {
 // as per
 // http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html#_Toc398718040
 uint16_t Adafruit_MQTT::publishPacket(uint8_t *packet, const char *topic,
-                                      uint8_t *data, uint16_t bLen, uint8_t qos,
-                                      uint16_t maxPacketLen) {
+                                      const uint8_t *data, uint16_t bLen,
+                                      uint8_t qos, uint16_t maxPacketLen) {
   uint8_t *p = packet;
   uint16_t len = 0;
 
@@ -857,7 +857,7 @@ bool Adafruit_MQTT_Publish::publish(const char *payload) {
 }
 
 // publish buffer of arbitrary length
-bool Adafruit_MQTT_Publish::publish(uint8_t *payload, uint16_t bLen) {
+bool Adafruit_MQTT_Publish::publish(const uint8_t *payload, uint16_t bLen) {
 
   return mqtt->publish(topic, payload, bLen, qos);
 }

Adafruit_MQTT.h

@@ -187,7 +187,7 @@ class Adafruit_MQTT {
   // Publish a message to a topic using the specified QoS level.  Returns true
   // if the message was published, false otherwise.
   bool publish(const char *topic, const char *payload, uint8_t qos = 0);
-  bool publish(const char *topic, uint8_t *payload, uint16_t bLen,
+  bool publish(const char *topic, const uint8_t *payload, uint16_t bLen,
                uint8_t qos = 0);
 
   // Add a subscription to receive messages for a topic.  Returns true if the
@@ -258,7 +258,7 @@ class Adafruit_MQTT {
   uint8_t connectPacket(uint8_t *packet);
   uint8_t disconnectPacket(uint8_t *packet);
   static uint16_t packetAdditionalLen(uint16_t currLen);
-  uint16_t publishPacket(uint8_t *packet, const char *topic, uint8_t *payload,
+  uint16_t publishPacket(uint8_t *packet, const char *topic, const uint8_t *payload,
                          uint16_t bLen, uint8_t qos, uint16_t maxPacketLen);
   uint8_t subscribePacket(uint8_t *packet, const char *topic, uint8_t qos);
   uint8_t unsubscribePacket(uint8_t *packet, const char *topic);
@@ -279,7 +279,7 @@ class Adafruit_MQTT_Publish {
               // This might be ignored and a higher precision value sent.
   bool publish(int32_t i);
   bool publish(uint32_t i);
-  bool publish(uint8_t *b, uint16_t bLen);
+  bool publish(const uint8_t *b, uint16_t bLen);
 
 private:
   Adafruit_MQTT *mqtt;