Github user zentol commented on the issue:
https://github.com/apache/flink/pull/4646
I went through all usages of `HttpHeaders.Names.CONTENT_TYPE` and they are
all related to rest operators, making `RestContants` a good place to put them.
Also, ConfigConstants should be reserved for constants related to the
configuration of Flink, which this isn't. Lastly, many of these usage will be
eclipsed in the near future anyway.---
