Yes, it does. See https://github.com/stackmob/newman/blob/3a3cec0cd94c16ab0adfb7bcc665ed5b3804b55b/src/main/scala/com/stackmob/newman/SprayHttpClient.scala#L86

It tries to parse the Content-Type from the passed-in Newman headers, and if so it writes that to the Content-Type on the HttpEntity. Failing the parse, it defaults to `application/json`.