add test for .isValid() function

test/test.PrivateKey.js

@@ -49,4 +49,11 @@ describe('PrivateKey', function() {
     a.network().name.should.equal('livenet');
   });
 
+  describe('#isValid', function() {
+    it('should detect this private key as valid', function() {
+      var privKey = new PrivateKey('5JQkUX6RNQC91xv4Www6Cgbxb6Eri6WBJjtPGqwfXVKBzT37cAf');
+      privKey.isValid().should.equal(true);
+    });
+  });
+ 
 });