temporarly fix an issue of double callback in JWS

package.json

@@ -19,6 +19,7 @@
     "url": "https://github.com/auth0/node-jsonwebtoken/issues"
   },
   "dependencies": {
+    "cb": "~0.1.0",
     "joi": "~6.10.1",
     "jws": "^3.0.0",
     "ms": "^0.7.1",

sign.js

@@ -2,6 +2,7 @@ var Joi = require('joi');
 var timespan = require('./lib/timespan');
 var xtend = require('xtend');
 var jws = require('jws');
+var cb = require('cb');
 
 var sign_options_schema = Joi.object().keys({
   expiresIn: [Joi.number().integer(), Joi.string()],
@@ -124,6 +125,8 @@ module.exports = function (payload, secretOrPrivateKey, options, callback) {
   var encoding = options.encoding || 'utf8';
 
   if (typeof callback === 'function') {
+    callback = callback && cb(callback).once();
+
     jws.createSign({
       header: header,
       privateKey: secretOrPrivateKey,

test/async_sign.tests.js

@@ -20,8 +20,8 @@ describe('signing a token asynchronously', function() {
 
     it('should work', function (done) {
       jwt.sign({abc: 1}, "secret", {}, function (err, res) {
-         expect(err).to.not.exist();
-         done();
+        expect(err).to.be.null();
+        done();
       });
     });