Fix errors thrown by Rack::Lint

[adamcrown]

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 site via prax in case anybody runs into the same issue.

[ysbaddaden]

Thanks! I'll look at it ASAP, and add Rack::Lint to the test suite.