Do not raise ArgumentError, key cannot be blank if user forge a blank session cookie

We issued this error when issing dalli_store/mem_cache_store as session_store (also happen here : http://stackoverflow.com/questions/17792893/rails4-dalli-argumenterror-key-cannot-be-blank/)

I used same cache_key method than ActionDispatch::Session::CacheStore.

I do not manage to find how to test ActionDispatch::Session::DalliStore that seems to share lot of code with Rack::Session::Dalli (aka ActionDispatch::Session::MemCacheStore superclass)