`Properties.json()` now converts Correlation Data bytes() objects to hex.

Closes #555. Thanks to Charles Smith.

ChangeLog.txt

@@ -33,6 +33,8 @@ v1.6.0 - 2021-xx-xx
   Closes #520.
 - Experimental support for Websockets continuation frames. Closes #500.
   Closes #89.
+- `Properties.json()` now converts Correlation Data bytes() objects to hex.
+  Closes #555.
 
 
 v1.5.1 - 2020-09-22

src/paho/mqtt/properties.py

@@ -319,7 +319,11 @@ class Properties(object):
         for name in self.names.keys():
             compressedName = name.replace(' ', '')
             if hasattr(self, compressedName):
-                data[compressedName] = getattr(self, compressedName)
+                val = getattr(self, compressedName)
+                if compressedName == 'CorrelationData' and isinstance(val, bytes):
+                    data[compressedName] = val.hex()
+                else:
+                    data[compressedName] = val
         return data
 
     def isEmpty(self):