synchronize on param filter cache.

Do we actually need this cache?

actionpack/lib/action_dispatch/http/filter_parameters.rb

@@ -1,3 +1,4 @@
+require 'mutex_m'
 require 'active_support/core_ext/hash/keys'
 require 'active_support/core_ext/object/duplicable'
 
@@ -20,7 +21,7 @@ module Http
     #   end
     #   => reverses the value to all keys matching /secret/i
     module FilterParameters
-      @@parameter_filter_for  = {}
+      @@parameter_filter_for  = {}.extend(Mutex_m)
 
       ENV_MATCH = [/RAW_POST_DATA/, "rack.request.form_vars"] # :nodoc:
       NULL_PARAM_FILTER = ParameterFilter.new # :nodoc:
@@ -64,7 +65,11 @@ def env_filter
       end
 
       def parameter_filter_for(filters)
-        @@parameter_filter_for[filters] ||= ParameterFilter.new(filters)
+        @@parameter_filter_for.synchronize do
+          # Do we *actually* need this cache? Constructing ParameterFilters
+          # doesn't seem too expensive.
+          @@parameter_filter_for[filters] ||= ParameterFilter.new(filters)
+        end
       end
 
       KV_RE   = '[^&;=]+'