lib/bugsnag/middleware/rack_request: early load session for Rails 4

Fixes #144 (https://github.com/bugsnag/bugsnag-ruby/issues/144)

Context: https://github.com/rails/rails/issues/10813
If I remove `if session.loaded?`, then potentially, we can get a ``NoMethodError: undefined method `to_hash'``. I just realised that I should add `session.respond_to?(:loaded?)` to avoid this.

Anyway, what I propose it to write it like this:

```ruby
notification.add_tab(:session, session.to_hash) if session.respond_to?(:to_hash?)
```

I assume that since the time of the creation of that code the `session` object has always been a `Hash` here. So if it's not a `Hash`, then it's probably a Rails 4 session.

The worst that can happen is that the session tab won't be included at all.