fix issue with tn-cli when fully formated json is not handled correctly

tn-cli/tn-cli.py

@@ -195,10 +195,13 @@ def attachment(filename):
         stdoutln("Error processing attachment '" + filename + "':", err)
         return None
 
-# encode_to_bytes takes an object/dictionary and converts it to json-formatted byte array.
+# encode_to_bytes converts the src to json-formatted byte array.
+# An object/dictionary is first converted to json, a string is directly converted to bytes.
 def encode_to_bytes(src):
     if src == None:
         return None
+    if isinstance(src, str):
+        return src.encode('utf-8')
     return json.dumps(src).encode('utf-8')
 
 # Parse credentials