Improve UTF-8 handling in avrorouter

The json_stringn function should be used instead of the json_string to
allow null characters as well as non-null terminated strings to be
embedded in the JSON values.

The CDC example Python programs now decode the data as UTF-8 instead of
ASCII.

server/modules/protocol/examples/cdc.py

@@ -37,7 +37,7 @@ def read_json():
             rbuf += buf
             while True:
                 rbuf = rbuf.lstrip()
-                data = decoder.raw_decode(rbuf.decode('ascii'))
+                data = decoder.raw_decode(rbuf.decode('utf_8'))
                 rbuf = rbuf[data[1]:]
                 print(json.dumps(data[0]))
         except ValueError as err: