Check session before start?

[diwms]

Hi,
I would like to contribute to this package.
I use this package in pair with slim/framework and slim/flash packages.

In my case the problem in SessionMiddleware - i got notice from php that session is already started. It's because registering in dependency injection container slim/flash package assume that session is started and ready to work.

So, my idea is - improve a little bit SessionMiddleware by wrapping session start

- $this->session->start();
+ if ($session->isStarted() === false) { 
        $session->start();
}

What do you think? In case of approve I'll open PR with changes.

[odan]

I think that makes sense and would be a good improvement. Please create a PR.