Fix errors thrown by Rack::Lint

According to Rack::Lint there are a couple small issues with prax's Rack spec validation. These were the errors that were being thrown.

    rack.input #<StringIO> does not have ASCII-8BIT as its external encoding

and

    env variable SERVER_PORT has non-string value

If you'd like to test it yourself here's the simple `config.ru` I used.

    require 'rack/lint'
    use Rack::Lint
    run ->(*) { [200, {'Content-Type' => 'text/html'}, ['Lint?']] }

I ran into these while trying to run a [Middleman](http://middlemanapp.com) site via prax in case anybody runs into the same issue.