[message] ensure decoding can be done as string->dict too

graphenecommon/message.py

@@ -254,7 +254,11 @@ def verify(self, **kwargs):
             :returns: True if the message is verified successfully
             :raises InvalidMessageSignature if the signature is not ok
         """
-        assert isinstance(self.message, dict), "Message must be dictionary"
+        if not isinstance(self.message, dict):
+            try:
+                self.message = json.loads(self.message)
+            except Exception:
+                raise ValueError("Message must be valid JSON")
 
         payload = self.message.get("payload")
         assert payload, "Missing payload"

tests/test_message.py

@@ -1,4 +1,5 @@
 # -*- coding: utf-8 -*-
+import json
 import unittest
 from .fixtures import fixture_data, Message, MessageV1, MessageV2
 from pprint import pprint
@@ -48,6 +49,12 @@ def test_v2_enc(self):
         v = MessageV2(c)
         v.verify()
 
+    def test_v2_enc_string(self):
+        m = MessageV2("foobar")
+        c = m.sign(account="init0")
+        v = MessageV2(json.dumps(c))
+        v.verify()
+
     def test_v2andv1_enc(self):
         m = MessageV2("foobar")
         c = m.sign(account="init0")