Change API protocol to fix json normalization

A dictionnary's order is not deterministic (eg. when switching python
versions), thus the previous protocol would often fail to authenticate a
legitimate request.

api/client/apiclient.py

@@ -33,7 +33,8 @@ def sendReq(url):
         mac = hmac.new(key.encode('utf-8'),
                        msg=str(int(time)).encode('utf-8'),
                        digestmod=hashlib.sha256)
-        mac.update(json.dumps(payload).encode('utf-8'))
+        payload_enc = json.dumps(payload)
+        mac.update(payload_enc.encode('utf-8'))
 
         auth = {
             'keyId': keyId,
@@ -43,7 +44,7 @@ def sendReq(url):
 
         return {
             'auth': auth,
-            'req': payload,
+            'req': payload_enc,
         }
 
     def decorator(fct):