fix: make parse_fieldset more resilient (#502)

Querys with `?fields[model]=` are already handled okay but when omitting `=` it results in an exception. This changes fixes that

Author: amaierhofer

lib/graphiti/query.rb

@@ -302,7 +302,7 @@ def parse_fieldset(fieldset)
       {}.tap do |hash|
         fieldset.each_pair do |type, fields|
           type = type.to_sym
-          fields = fields.split(",") unless fields.is_a?(Array)
+          fields = fields.to_s.split(",") unless fields.is_a?(Array)
           hash[type] = fields.map(&:to_sym)
         end
       end