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

@@ -298,10 +298,10 @@ 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, uint8_t qos) {
+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 = publishPacket(buffer, (uint16_t) sizeof(buffer), topic, data, bLen, qos);
   if (!sendPacket(buffer, len))
@@ -648,7 +648,7 @@ 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, uint16_t maxPacketLen, const char *topic,
-                                     uint8_t *data, uint16_t bLen, uint8_t qos) {
+                                      const uint8_t *data, uint16_t bLen, uint8_t qos) {
   uint8_t *p = packet;
   uint16_t len=2;           // control + length
 
@@ -825,7 +825,7 @@ 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

@@ -178,7 +178,7 @@ 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, uint8_t qos = 0);
+  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
   // subscription could be added or was already present, false otherwise.
@@ -244,7 +244,7 @@ class Adafruit_MQTT {
   uint8_t connectPacket(uint8_t *packet);
   uint8_t disconnectPacket(uint8_t *packet);
   uint16_t publishPacket(uint8_t *packet, uint16_t maxPacketLen,
-                         const char *topic, uint8_t *payload, uint16_t bLen, uint8_t qos);
+                         const char *topic, const uint8_t *payload, uint16_t bLen, uint8_t qos);
   uint8_t subscribePacket(uint8_t *packet, const char *topic, uint8_t qos);
   uint8_t unsubscribePacket(uint8_t *packet, const char *topic);
   uint8_t pingPacket(uint8_t *packet);
@@ -262,7 +262,7 @@ 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: