Reinstate CloudFlare support + custom protocol

- Reinstating custom support for CF-VISITOR CloudFlare header +
  options[:protocol]

- Fixing failing tests
  Instead of passing in `X-Forwarded-Proto` to the Rack env, we can now
  safely rely on `HTTP_` env variables which are automatically set. Per
  https://github.com/rack/rack/blob/master/SPEC.rdoc#the-environment-:

      HTTP_ Variables:
      Variables corresponding to the client-supplied HTTP request headers
      (i.e., variables whose names begin with HTTP_). The presence or absence
      of these variables should correspond with the presence or absence of the
      appropriate HTTP header in the request. See RFC3875 section 4.1.18 for
      specific behavior.

lib/prerender_rails.rb

@@ -181,9 +181,16 @@ def get_prerendered_page_response(env)
       end
     end
 
+    def get_cloudflare_scheme(env)
+      match = /"scheme":"(http|https)"/.match(env['CF-VISITOR'])
+      match && match[1]
+    end
+
     def build_api_url(env)
       request_obj = Rack::Request.new(env)
-      url = "#{request_obj.scheme}://#{request_obj.host}#{request_obj.fullpath}"
+      scheme = @options[:protocol] || get_cloudflare_scheme(env) || request_obj.scheme
+      url = "#{scheme}://#{request_obj.host}#{request_obj.fullpath}"
+
       prerender_url = get_prerender_service_url()
       forward_slash = prerender_url[-1, 1] == '/' ? '' : '/'
       "#{prerender_url}#{forward_slash}#{url}"

test/lib/prerender_rails.rb

@@ -200,15 +200,15 @@
 
     # Check X-Forwarded-Proto because Heroku SSL Support terminates at the load balancer
     it "should build the correct api url for the Heroku SSL Addon support with single value" do
-      request = Rack::MockRequest.env_for "http://google.com/search?q=javascript", { 'X-FORWARDED-PROTO' => 'https'}
+      request = Rack::MockRequest.env_for "http://google.com/search?q=javascript", { 'HTTP_X_FORWARDED_PROTO' => 'https'}
       ENV['PRERENDER_SERVICE_URL'] = nil
       assert_equal 'http://service.prerender.io/https://google.com/search?q=javascript', @prerender.build_api_url(request)
     end
 
 
     # Check X-Forwarded-Proto because Heroku SSL Support terminates at the load balancer
     it "should build the correct api url for the Heroku SSL Addon support with double value" do
-      request = Rack::MockRequest.env_for "http://google.com/search?q=javascript", { 'X-FORWARDED-PROTO' => 'https,http'}
+      request = Rack::MockRequest.env_for "http://google.com/search?q=javascript", { 'HTTP_X_FORWARDED_PROTO' => 'https,http'}
       ENV['PRERENDER_SERVICE_URL'] = nil
       assert_equal 'http://service.prerender.io/https://google.com/search?q=javascript', @prerender.build_api_url(request)
     end