fix: wrong HTTP header to detect the websocket request

Author: did

lib/maglev/preview_constraint.rb

@@ -33,7 +33,7 @@ def crawler?(request)
     end
 
     def websocket?(request)
-      request.headers['Upgrade'] == 'websocket'
+      request.headers['HTTP_UPGRADE'] == 'websocket'
     end
 
     def match_host?(request)