Don't create an AX extension object unless necessary

This is a workaround for a bug in the ruby-openid gem which outputs a
"mode" parameter even if there are no other values being queried. This
causes some providers (such as Google) to prompt the user differently
(for example, Google says "site X is requesting information from your
Google account" instead of just "sign in to site X with your Google
account").

lib/rack/openid.rb

@@ -238,12 +238,19 @@ def add_attribute_exchange_fields(oidreq, fields)
         axreq = ::OpenID::AX::FetchRequest.new
 
         required = Array(fields['required']).select(&URL_FIELD_SELECTOR)
-        required.each { |field| axreq.add(::OpenID::AX::AttrInfo.new(field, nil, true)) }
-
         optional = Array(fields['optional']).select(&URL_FIELD_SELECTOR)
-        optional.each { |field| axreq.add(::OpenID::AX::AttrInfo.new(field, nil, false)) }
 
-        oidreq.add_extension(axreq)
+        if required.any? || optional.any?
+          required.each do |field|
+            axreq.add(::OpenID::AX::AttrInfo.new(field, nil, true))
+          end
+
+          optional.each do |field|
+            axreq.add(::OpenID::AX::AttrInfo.new(field, nil, false))
+          end
+
+          oidreq.add_extension(axreq)
+        end
       end
 
       def add_oauth_fields(oidreq, fields)