Some OpenSearch REST APIs will return a Content-Type of text/plain for certain
status codes which will trigger logging when the response is not parsable as
JSON. Checking the content-type first avoids this error.
This frequently happens with 401s, which occurs when we rotate user keys.
The input that triggers this behaviour can be triggered with `curl`:
```
$ curl -k -u"admin:wrong" -w"\n%{http_code}\n%{content_type}\n" https://localhost:9200/_cluster/health
Unauthorized
401
text/plain; charset=UTF-8
```
Signed-off-by: Trevor McCulloch <[email protected]>
