Fix default inferrer being frozen.

Author: beauby

lib/jsonapi/rails/controller.rb

@@ -87,14 +87,14 @@ def deserializable_resource(key, options = {}, &block)
       # Overridden by the `class` renderer option.
       # @return [Hash{Symbol=>Class}]
       def jsonapi_class
-        JSONAPI::Rails.config[:jsonapi_class]
+        JSONAPI::Rails.config[:jsonapi_class].dup
       end
 
       # Hook for serializable class mapping (for errors).
       # Overridden by the `class` renderer option.
       # @return [Hash{Symbol=>Class}]
       def jsonapi_errors_class
-        JSONAPI::Rails.config[:jsonapi_errors_class]
+        JSONAPI::Rails.config[:jsonapi_errors_class].dup
       end
 
       # Hook for the jsonapi object.