"Incompatible character encodings: ASCII-8BIT and UTF-8" somewhere in Rack's libraries

Here is the Rackup file to reproduce:

```
# Here is the workaround:
=begin
require 'puma'
class Puma::Configuration
  def inspect
    "(not available)"
  end
end
=end

run(lambda { |env| ['200', {'Content-Type' => 'text/html'}, ["Hi!"]] })
```

This is in Ruby 1.9.3.
