Allow jwt to take null or undefined

Minor change, but I've had to do this in my code which I'm not a big fan of:

`api.jwt(getToken().accessToken || null)`

src/index.js

@@ -284,7 +284,7 @@ class Frisbee {
   }
 
   jwt(token) {
-    if (token === null) delete this.headers.Authorization;
+    if (token === null || token === undefined) delete this.headers.Authorization;
     else if (typeof token === 'string')
       this.headers.Authorization = `Bearer ${token}`;
     else throw new TypeError('jwt token must be a string');