fix Wallet default constructor

src/wallet.js

@@ -34,7 +34,7 @@ var Wallet = function (seed, options) {
     this.newMasterKey = function(seed, network) {
         if (!seed) {
             var seed= new Array(32);
-            rng.nextBytes(seedBytes);
+            rng.nextBytes(seed);
         }
         masterkey = new HDNode(seed, network);
     }

test/wallet.js

@@ -39,6 +39,13 @@ describe('Wallet', function() {
       assert.equal(account.depth, 2)
     })
 
+    describe('when seed is not specified', function(){
+      it('generates a seed', function(){
+        var wallet = new Wallet()
+        assert.ok(wallet.getMasterKey())
+      })
+    })
+
     describe('constructor options', function() {
       var wallet;
       beforeEach(function() {