PrivateKey: Use defaultNetwork if not specified

lib/privatekey.js

@@ -40,7 +40,7 @@ var PrivateKey = function PrivateKey(data, network, compressed) {
 
   var info = {
     compressed: typeof(compressed) !== 'undefined' ? compressed : true,
-    network: network || 'livenet'
+    network: network || networks.defaultNetwork.name
   };
 
   // detect type of data
@@ -116,9 +116,9 @@ PrivateKey._transformBuffer = function(buf, network, compressed) {
   }
 
   if (buf[0] === networks.livenet.privatekey) {
-    info.network = 'livenet';
+    info.network = networks.livenet.name;
   } else if (buf[0] === networks.testnet.privatekey) {
-    info.network = 'testnet';
+    info.network = networks.testnet.name;
   } else {
     throw new Error('Invalid network');
   }