Fixed session to xml inspector for Rails4 ActionDispatch Session.

ActionDispatch 4.0.0(beta1) Request::Session don't have ```each``` method.

```
undefined method `each' for #<ActionDispatch::Request::Session:0x007ff5ef980110>
 # /Users/yuichi-tateno/git/github/rack_session_access/lib/rack_session_access/middleware.rb:54:in `block (2 levels) in show'
```