make sure we don't set env["airbrake.error_id"] for async notices

Setting this makes no sense, since this value is only used for informing
the end user about the ID of the exception that she experienced. If
Airbrake is used asynchronously, notice will probably be sent in the
background, after the response.

Closing airbrake#154

lib/airbrake.rb

@@ -138,6 +138,7 @@ def send_notice(notice)
       if configuration.public?
         if configuration.async?
           configuration.async.call(notice)
+          nil # make sure we never set env["airbrake.error_id"] for async notices
         else
           sender.send_to_airbrake(notice)
         end

lib/airbrake/user_informer.rb

@@ -10,6 +10,7 @@ def replacement(with)
 
     def call(env)
       status, headers, body = @app.call(env)
+
       if env['airbrake.error_id'] && Airbrake.configuration.user_information
         new_body = []
         replace  = replacement(env['airbrake.error_id'])
@@ -20,6 +21,7 @@ def call(env)
         headers['Content-Length'] = new_body.sum(&:bytesize).to_s
         body = new_body
       end
+
       [status, headers, body]
     end
   end