Respect custom content-type header in the reactive client.

[sothawo]

In order to be able to use the current reactive client against a version 8 cluster, it is necessary to set the following headers:

"Accept: application/vnd.elasticsearch+json;compatible-with=7"
"Content-Type: application/vnd.elasticsearch+json;compatible-with=7"

When doing this by configuring the client, the current code in addition adds the

"Content-Type: application/json"

header which leads to an error returned from Elasticsearch.

A content-type header determined by the content should only be added when no header was set by the user.