Fix Rack Protocol implementation

From [Rack SPEC](http://rack.rubyforge.org/doc/SPEC.html) :
- The Body must respond to each and must only yield String values.
- The keys must contain at least one dot, and should be prefixed uniquely.
    > 'warden' => 'warden.env'

Obviously, this breaks compatibility with existing applications.

Please note that the new key is a proposal.