Index throws exception due to bug in parseheader

[kuroburki]

Code looks for string "Content-Length" in the response header, but received header shows "content-length".

[hthinh]

I got the same problem for elasticsearch 6.2.0, quick fix is modify it to look for "content-length". However, HTTP protocol states that: headers are case insensitive (RFC 7230) so it should be handled properly.