Make WebxmlOpenStruct work on jruby 9.2 and 9.3

I believe this doesn't change any behaviour on either version (although
weird behaviour may exist when users set keys with string values
explicitly).

Author: joerixaop

lib/warbler/traits/war.rb

@@ -181,16 +181,6 @@ def initialize(key = 'webxml')
           @servlet_filter_async = nil # true/false
         end
 
-        def [](key)
-          new_ostruct_member!(key)
-          send(key)
-        end
-
-        def []=(key, value)
-          new_ostruct_member!(key)
-          send("#{key}=", value)
-        end
-
         def servlet_filter; @servlet_filter ||= 'org.jruby.rack.RackFilter' end
         attr_writer :servlet_filter
 
@@ -248,7 +238,7 @@ def method_missing(mid, *args)
             if len != 1
               raise ArgumentError, "wrong number of arguments (#{len} for 1)", caller(1)
             end
-            modifiable[new_ostruct_member!(mname)] = args[0]
+            self[mname] = args[0]
           elsif len == 0
             @table[mid]
           else