mqtt: once again, do not quote numbers in MQTT JSON payload

Restore behaviour from dd12251e43
7ba1283024 unintentionally removed it
Instead of reverting to the 'atof' code, just use the provided string
as-is via RawJson (aka `serialized()` in the v6)

Note that the old approach may have caused some silent data loss b/c the
data could have been stored in 'double' or 'u64', where the ArduinoJson
only stores 'u32' and 'float' in it's internal buffer and does not (really)
warn us about any type incompatibilities.
(at least by default. but, the alternative is to store all floating point
numbers as 'double' internally, see ArduinoJson/Configuration.hpp)

resolves #2503

code/espurna/mqtt.cpp

@@ -1424,8 +1424,18 @@ void mqttFlush() {
     root[MQTT_TOPIC_MESSAGE_ID] = (Rtcmem->mqtt)++;
 #endif
 
+    // ref. https://github.com/xoseperez/espurna/issues/2503
+    // pretend that the message is already a valid json value
+    // when the string looks like a number
+    // ([0-9] with an optional decimal separator [.])
     for (auto& payload : _mqtt_json_payload) {
-        root[payload.topic().c_str()] = payload.message().c_str();
+        const char* const topic { payload.topic().c_str() };
+        const char* const message { payload.message().c_str() };
+        if (isNumber(payload.message())) {
+            root[topic] = RawJson(message);
+        } else {
+            root[topic] = message;
+        }
     }
 
     String output;

code/espurna/utils.cpp

@@ -229,43 +229,41 @@ double roundTo(double num, unsigned char positions) {
     return round(num * multiplier) / multiplier;
 }
 
-bool isNumber(const String& value) {
-    if (value.length()) {
-        const char* begin { value.c_str() };
-        const char* end { value.c_str() + value.length() };
-
-        bool dot { false };
-        bool digit { false };
-        const char* ptr { begin };
+bool isNumber(const char* begin, const char* end) {
+    bool dot { false };
+    bool digit { false };
 
-        while (ptr != end) {
-            switch (*ptr) {
-            case '\0':
-                break;
-            case '-':
-            case '+':
-                if (ptr != begin) {
-                    return false;
-                }
-                break;
-            case '.':
-                if (dot) {
-                    return false;
-                }
-                dot = true;
-                break;
-            case '0' ... '9':
-                digit = true;
-                break;
-            case 'a' ... 'z':
-            case 'A' ... 'Z':
+    for (auto ptr = begin; ptr != end; ++ptr) {
+        switch (*ptr) {
+        case '\0':
+            break;
+        case '-':
+        case '+':
+            if (ptr != begin) {
                 return false;
             }
-
-            ++ptr;
+            break;
+        case '.':
+            if (dot) {
+                return false;
+            }
+            dot = true;
+            break;
+        case '0' ... '9':
+            digit = true;
+            break;
+        case 'a' ... 'z':
+        case 'A' ... 'Z':
+            return false;
         }
+    }
 
-        return digit;
+    return digit;
+}
+
+bool isNumber(const String& value) {
+    if (value.length()) {
+        return isNumber(value.begin(), value.end());
     }
 
     return false;

code/espurna/utils.h

@@ -41,6 +41,7 @@ bool sslFingerPrintArray(const char * fingerprint, unsigned char * bytearray);
 bool sslFingerPrintChar(const char * fingerprint, char * destination);
 
 char* strnstr(const char* buffer, const char* token, size_t n);
+bool isNumber(const char* begin, const char* end);
 bool isNumber(const String&);
 
 uint32_t randomNumber();