Fix tx.creates

Author: heikoheiko

ethereum/transactions.py

@@ -136,7 +136,7 @@ def log_dict(self):
     @property
     def creates(self):
         "returns the address of a contract created by this tx"
-        if self.to == '':
+        if self.to in (b'', '\0'*20):
             return mk_contract_address(self.sender, self.nonce)