Added test if session is started before using SessionStorage.

Hi @Nyholm 

I like your library alot, it helps me built my project. However I have noticed issue if I don't have a php session started, there is following error showing on:

```
Fatal error: Uncaught Exception: CSRF state token does not match one provided. thrown in /var/www/html/filipgorny/vendor/happyr/linkedin-api-client/src/HappyR/LinkedIn/LinkedIn.php on line 300
```

I have resolved it by the following commit and I thought you may consider it to merge.