Remove to parameter in publishing contract transaction

If sending empty `to` parameter, the error `Invalid params: expected a hex-encoded hash with 0x prefix.` is raised by Parity node.

src/Contract.php

@@ -397,7 +397,6 @@ class Contract
             if (count($arguments) > 0) {
                 $transaction = $arguments[0];
             }
-            $transaction['to'] = '';
             $transaction['data'] = '0x' . $this->bytecode . Utils::stripZero($data);
 
             $this->eth->sendTransaction($transaction, function ($err, $transaction) use ($callback){
@@ -622,4 +621,4 @@ class Contract
             return $functionData;
         }
     }
-}
+}