Use correct user-association endpoint for Chef 12

The correct endpoint for user-association is:

POST /organizations/ORGNAME/users

The API currently being made will be rejected with a 405 Method Not
Allowed error.

lib/chef/provider/chef_organization.rb

@@ -52,7 +52,7 @@ def whyrun_supported?
     new_resource.members.each do |user|
       if !existing_members.include?(user)
         converge_by "Add #{user} to organization #{new_resource.name}" do
-          rest.post("#{rest.root_url}/organizations/#{new_resource.name}/users/#{user}", {})
+          rest.post("#{rest.root_url}/organizations/#{new_resource.name}/users/", { 'username' => user })
         end
       end
     end

lib/chef/resource/chef_organization.rb

@@ -35,10 +35,11 @@ def invites_specified?
     !!@invites
   end
 
-  # A list of users who must be members of the org.  This will use the new Chef 12
-  # POST /organizations/ORG/users/NAME endpoint to add them directly to the org.
-  # If you do not have permission to perform this operation, and the users are not
-  # a part of the org, the resource update will fail.
+  # A list of users who must be members of the org.  This will use the
+  # new Chef 12 POST /organizations/ORG/users endpoint to add them
+  # directly to the org.  If you do not have permission to perform
+  # this operation, and the users are not a part of the org, the
+  # resource update will fail.
   def members(*users)
     if users.size == 0
       @members || []