Refactor attr_reader method of ZenTao query

Narrow scope of "attr_reader" methods.
They are only used in current class, so put them under private.

lib/gitlab/zentao/query.rb

@@ -7,8 +7,6 @@ module Gitlab
       ISSUES_DEFAULT_LIMIT = 20
       ISSUES_MAX_LIMIT = 50
 
-      attr_reader :client, :params
-
       def initialize(integration, params)
         @client = Client.new(integration)
         @params = params
@@ -32,6 +30,8 @@ module Gitlab
 
       private
 
+      attr_reader :client, :params
+
       def query_options
         {
           order: query_order,