Make headers case-insensitive

[mayyamus]

Elasticsearch version:all

Plugins installed: []

JVM version:all

OS version:all

Description of the problem including expected versus actual behavior:
perhaps setHeaders method can't override default header in RestClient, because httpHeader names are case-insensitive.suggest converting all default header and request header names to lowercase/uppercase.

[adityasrini]

I’d like to take this up if no one is already working on this or has been assigned to this.

[javanna]

go ahead @adityasrini thanks!

[javanna]

given that we have decided (see #30616) to remove support for default headers, as they are already supported by the apache http client which we use to perform requests, I am going to close this.