Reset Capybara.app_host after each spec

Workaround for Capybara integration specs which fail to do the same.

spec/spec_helper.rb

@@ -17,10 +17,6 @@ $:.detect do |dir|
   end
 end
 
-RSpec.configure do |c|
-  c.filter_run_excluding :skip_on_windows => !(RbConfig::CONFIG['host_os'] =~ /mingw32/).nil?
-end
-
 require 'capybara/webkit'
 connection = Capybara::Webkit::Connection.new(:socket_class => TCPSocket)
 $webkit_browser = Capybara::Webkit::Browser.new(connection)
@@ -35,6 +31,11 @@ Capybara.register_driver :reusable_webkit do |app|
   Capybara::Webkit::Driver.new(app, :browser => $webkit_browser)
 end
 
+RSpec.configure do |c|
+  c.filter_run_excluding :skip_on_windows => !(RbConfig::CONFIG['host_os'] =~ /mingw32/).nil?
+  c.before { Capybara.app_host = nil }
+end
+
 def with_env_vars(vars)
   old_env_variables = {}
   vars.each do |key, value|