fixed PrivateKey initialization from JSON

lib/privatekey.js

@@ -247,9 +247,10 @@ PrivateKey._transformJSON = function(json) {
     json = JSON.parse(json);
   }
   var bn = new BN(json.bn, 'hex');
+  var network = Networks.get(json.network);
   return {
     bn: bn,
-    network: json.network,
+    network: network,
     compressed: json.compressed
   };
 };