Fix custom filters assignment (#48)

Author: Ruslan Khaertdinov

lib/jsonapi/utils/support/filter/custom.rb

@@ -11,12 +11,8 @@ def custom_filters(*attrs)
     def custom_filter(attr)
       attr = attr.to_sym
       @_allowed_filters[attr] = {}
-
-      if !@_allowed_custom_filters.is_a?(Array)
-        @_allowed_custom_filters = Array(attr)
-      elsif @_allowed_custom_filters.include?(attr)
-        @_allowed_custom_filters.push(attr)
-      end
+      @_allowed_custom_filters ||= []
+      @_allowed_custom_filters |= [attr]
     end
   end
 end