fix: added keypair.seed() secret param on Builder instance. (#2)

mobius_client_python/auth/challenge.py

@@ -10,12 +10,14 @@
 import random
 import datetime
 
+
 # Helper object for timestamps
 class Time(object):
     def __init__(self, min_time=0, max_time=0):
         self.minTime = int(min_time)
         self.maxTime = int(max_time)
 
+
 # Generates challenge transaction on developer's side.
 class Challenge(object):
     def __init__(self, developer_secret, expires_in):
@@ -30,15 +32,18 @@ def call(self):
         keypair = self.keypair()
 
         builder = Builder(address=keypair.address().decode(),
-                          sequence=self.random_sequence())
+                          sequence=self.random_sequence(),
+                          secret=keypair.seed())
+
+        builder.sign()
 
         builder.add_memo(self.memo())
         builder.add_time_bounds(self.build_time_bounds())
 
         builder.append_payment_op(source=Keypair.random(),
                                   destination=keypair.address().decode(),
                                   amount='0.000001')
-        builder.sign(keypair.seed())
+        builder.sign()
 
         te = builder.gen_te()