Deprecation warning when having charset in the request Content-Type

[Pyppe]

Elasticsearch version: 5.6.3

Plugins installed: []

JVM version: 1.8.0_144

OS version: macOS Sierra 10.12.6 (Darwin Kernel Version 16.7.0)

Description of the problem including expected versus actual behavior:
We get a deprecation warning if using Content-Type: application/x-ndjson; charset=UTF-8 header for the Bulk API. That is, the server only checks if the Content-Type === "application/x-ndjson" and otherwise complains about missing Content-Type.

Steps to reproduce:

curl -XPOST 'localhost:9200/_bulk?pretty' -H 'Content-Type: application/x-ndjson; charset=UTF-8' -d'
{ "index" : { "_index" : "test", "_type" : "type1", "_id" : "1" } }
{ "field1" : "value1" }
'

Provide logs (if relevant):

[WARN ][o.e.d.r.RestController   ] Content type detection for rest requests is deprecated. Specify the content type using the [Content-Type] header.

[Pyppe]

Opened this issue as a result of #22769 (comment).

[javanna]

We should definitely not issue a deprecation warning when Content-Type includes the charset. Another story is what we do with the charset, see #22769 .

[ebadyano]

Closing as 5.x isn't maintained anymore and the behaviour on 6.x is tracked under #28123