Make respond_to? work for all keys

On JRuby 9.3 new_ostruct_member! checks if it already knows a new key by
checking the `@table` instance variable, before checking whether a
method is already defined (I assume because it's faster in the common
case), the way `@table` is handled with auto-vivifying values made this
not work properly for nested assignments.

E.g. `config.webxml.rails.env = 'production'` would auto-vivify
config.webxml.rails without defining a singleton method.

On JRuby 9.3 we could scrap method_missing, it has the right behaviour
(i.e. the workaround for #366 added in 9245f4c is no longer
required). Unfortunately it's still required on JRuby 9.2

lib/warbler/traits/war.rb

@@ -176,7 +176,10 @@ class WebxmlOpenStruct < OpenStruct
 
         def initialize(key = 'webxml')
           @key = key
-          @table = Hash.new { |h, k| h[k] = WebxmlOpenStruct.new(k) }
+          @table = Hash.new { |h, k|
+            new_ostruct_member!(k)
+            h[k] = WebxmlOpenStruct.new(k)
+          }
 
           @servlet_filter_async = nil # true/false
         end