remove default expiry in jwt token

lib/util/jwtutil.js

@@ -1,17 +1,21 @@
 var assert = require('assert')
 var jws = require('jws')
+var _ = require('lodash')
 
 module.exports.encode = function(options) {
   assert.ok(options.payload, 'payload required')
   assert.ok(options.secret, 'secret required')
 
-  var expiry = options.expiry || Date.now() + 24 * 3600
+  var header = {
+    alg: 'HS256'
+  }
+
+  if (options.header) {
+    header = _.merge(header, options.header)
+  }
 
   return jws.sign({
-    header: {
-      alg: 'HS256'
-    , exp: expiry
-    }
+    header: header
   , payload: options.payload
   , secret: options.secret
   })