Are you asking about the headers I listed in https://github.com/stackmob/newman/pull/66/files#L0R102 ? Or some other headers?

The headers in that list are Spray's "protected" headers, the ones that are set automatically and internally by Spray. I could make it so that it tries to set them appropriately, if any one of them is passed. I'm not sure how much access I have to some of them, though.

The alternative is to override Spray's protected-ness, for which they include a mechanism but it is less preferred (per Spray's docs).