Yo dawg, I put a revert in ur revert

Readding support for specifying host for mock rack session

This reverts commit 62bd215a47.

lib/capybara.rb

@@ -15,7 +15,7 @@ module Capybara
   class InfiniteRedirectError < TimeoutError; end
   
   class << self
-    attr_accessor :debug, :asset_root, :app_host, :run_server
+    attr_accessor :debug, :asset_root, :app_host, :run_server, :default_host
     attr_accessor :default_selector, :default_wait_time, :ignore_hidden_elements
 
     def default_selector

lib/capybara/driver/rack_test_driver.rb

@@ -216,6 +216,10 @@ class Capybara::Driver::RackTest < Capybara::Driver::Base
 
 private
 
+  def build_rack_mock_session # :nodoc:
+    Rack::MockSession.new(app, Capybara.default_host)
+  end
+
   def current_path
     request.path rescue ""
   end