lib-http: http_client_request_add_header() - Replace existing header

lib-http: http_client_request_add_header() - Replace existing header

If header with the same key already exists, just replace the value.
HTTP supports having multiple headers with the same key only when they
can be rewritten into a single comma-separated header. So practically
there's no reason for lib-http to need to support adding multiple
headers. Replacing an existing value is more useful generally.