[fluentd http] the defaut value HTTP content type application/x-ndjson is unsupported on datadog

Content_type "application/x-ndjson" is generated by default. this parameter causes two issues.
1) The default value is not supported by datadog
2) The default value can not be overwritten by header

<label @DATADOG_ANLI>
<match **>
@type http
endpoint https://http-intake.logs.us5.datadoghq.com/api/v2/logs
http_method post
content_type "application/x-ndjson"
headers

.....
</match>
</label>

Step to Reproduce:

1. Forward logs to datadog over http

cat <<EOF|oc create -f -
apiVersion: "logging.openshift.io/v1"
kind: "ClusterLogging"
metadata:
  name: "instance"
  namespace: openshift-logging
spec:
  managementState: "Managed"
  collection:
    type: "fluentd"

apiVersion: logging.openshift.io/v1
kind: ClusterLogForwarder
metadata:
  name: instance
  namespace: openshift-logging
spec:
  outputs:
  - http:
      headers:
        DD-API-KEY: xxxxxx
        Content-Type: application/json
      method: POST
    name: datadog-anli
    type: http
    url: https://http-intake.logs.us5.datadoghq.com/api/v2/logs
  pipelines:
  - inputRefs:
    - application
    name:  to-datadog
    outputRefs:
    - datadog-anli
EOF

2. Check the collect pod logs

Actual result:

The datadog return message 'error="415 Unsupported Media Type'

2023-03-15 10:15:50 +0000 [warn]: suppressed same stacktrace
2023-03-15 10:15:50 +0000 [warn]: disable_chunk_backup is true. 5f6eda0d82a8828c1c2aa964cba76dda chunk is thrown away
2023-03-15 10:15:51 +0000 [warn]: got unrecoverable error in primary and no secondary error_class=Fluent::UnrecoverableError error="415 Unsupported Media Type {\"errors\":[

]}"

Expected result:

CLO don't set "application/x-ndjson" as the default Content-Type.

<label @DATADOG_ANLI>
<match **>
@type http
endpoint https://http-intake.logs.us5.datadoghq.com/api/v2/logs
http_method post
content_type "application/x-ndjson"
headers

.....
</match>