Fix following of absolute redirect URL in Sinatra

What's going on when the app redirects to an absolute URL?

* Sinatra relies on Rack::MockSession which sets SERVER_NAME to
  "example.org" [1] on request it makes. However, Webrat expects
  it to be "www.example.com"

* In MyClassyApp, the redirect URL is made out of Rack::Request#url [2]
  which uses  SERVER_NAME, which is set to "example.org" by Rack::MockSession.
  As a result, Webrat see it as an external redirect and don't follow it.

NOTE:

* SERVER_NAME is stricly equivalent to HTTP_HOST [3]
* This could have been fixed in Webrat::Session too. I'am not sure
  that it won't affect other frameworks so I left it intact.

      def request_page(url, http_method, data) #:nodoc:
         h = headers
         h['HTTP_REFERER'] = @current_url if @current_url
  +      h['HTTP_HOST']    = 'www.example.com'

[1] http://github.com/rack/rack/blob/5c00dd698edb953b4bee432fa12a20ba69a067c1/lib/rack/mock.rb#L79
[2] http://github.com/rack/rack/blob/5c00dd698edb953b4bee432fa12a20ba69a067c1/lib/rack/request.rb#L201
[3] http://github.com/rack/rack/blob/5c00dd698edb953b4bee432fa12a20ba69a067c1/lib/rack/request.rb#L72

lib/webrat/sinatra.rb

@@ -30,6 +30,7 @@ def #{verb}(path, data, headers = {})
             data[key] = Rack::Utils.unescape(value)
             data
           end
+          headers["HTTP_HOST"] = "www.example.com"
           @browser.#{verb}(path, params, headers)
         end
       RUBY

spec/integration/sinatra/modular_app.rb

@@ -5,4 +5,12 @@ class MyModularApp < Sinatra::Default
   get "/" do
     "Hello World"
   end
+
+  get "/redirect_absolute_url" do
+    redirect URI.join(request.url, "foo").to_s
+  end
+
+  get "/foo" do
+    "spam"
+  end
 end

spec/integration/sinatra/test/modular_app_test.rb

@@ -13,4 +13,9 @@ def test_it_works
     visit "/"
     assert_contain "Hello World"
   end
+
+  def test_redirects
+    visit "/redirect_absolute_url"
+    assert_equal "spam", response_body
+  end
 end