Remove duplicate kafka headers

Overview:

List<KafkaHeader> kafkaHeaders have to contains only unique values.

Steps to Reproduce:

Check result of KafkaHeaderUtils.kafkaHeadersFromMultiMap(params.getHeaders()) in RecordsPublishingServiceImpl.sendRecords(...)

Expected Results:

x-okapi-request-timestamp = <some value>
jobExecutionId = <some value>
........
correlationId = <some value>
x-okapi-request-ip = <some value>
Content-type = application/json
Accept = application/json, text/plain

Actual Results:
x-okapi-request-timestamp = <some value>
jobExecutionId = <some value>
........
correlationId = <some value>
x-okapi-request-ip = <some value>
Content-type = application/json
Accept = application/json, text/plain
Content-type = application/json
Accept = application/json, text/plain
Content-type = application/json
Accept = application/json, text/plain
Content-type = application/json
Accept = application/json, text/plain
Content-type = application/json
Accept = application/json, text/plain
Content-type = application/json
Accept = application/json, text/plain
Content-type = application/json
Accept = application/json, text/plain
Content-type = application/json
Accept = application/json, text/plain