Optimize index or any other operation on index failed with 406

(from github.com/rhayun)
i am using fess latest version 12.0 with Elasticsearch 6.1.1

when i try to do anything with index in the dashboard i get failed with this message

Content-Type header [application/x-www-form-urlencoded] is not supported

i believe its something with elasticsearch version 6 but cant find how to fix it
will appreciate your help


(from github.com/marevol)
Fess uses forked kopf. The source code is fess-6.0 branch.
Any contributions are welcome.