Add agent Set Group Membership as Default

https://developer.zendesk.com/rest_api/docs/core/group_memberships#set-m embership-as-default

Author: pchhetri

lib/zendesk_api/resources.rb

@@ -645,6 +645,10 @@ class TopicComment < TopicComment
  include Read
  end
 
+ class GroupMembership < Resource
+ put :make_default
+ end
+
  class Identity < Resource
  # Makes this identity the primary one bumping all other identities down one
  put :make_primary