Supports null values

I was having problems because my `this.req.body` is `null`, not `undefined` so i keep getting the `Only one method may be used to authenticate at a time` error.

Author: Volox

lib/authorise.js

@@ -59,8 +59,7 @@ function getBearerToken (done) {
     postToken = this.req.body && this.req.body.access_token;
 
   // Check exactly one method was used
-  var methodsUsed = (headerToken !== undefined) + (getToken !== undefined) +
-    (postToken !== undefined);
+  var methodsUsed = (!!headerToken) + (!!getToken) + (!!postToken);
 
   if (methodsUsed > 1) {
     return done(error('invalid_request',