Avoid hash lookups for building an array of required defaults

Only set the value once after it's calculated.

actionpack/lib/action_dispatch/routing/mapper.rb

@@ -176,12 +176,13 @@ def normalize_conditions!
               end
             end
 
-            @conditions[:required_defaults] = []
+            required_defaults = []
             options.each do |key, required_default|
               unless segment_keys.include?(key) || IGNORE_OPTIONS.include?(key) || Regexp === required_default
-                @conditions[:required_defaults] << key
+                required_defaults << key
               end
             end
+            @conditions[:required_defaults] = required_defaults
 
             via_all = options.delete(:via) if options[:via] == :all