Respect Spray Protected Headers

In creating Spray `HttpRequest`s in the `SprayHttpClient`, there are a subset of headers that should not be manually set. For example, `Content-Type` should be set via methods on the `HttpEntity`, rather than as a separate header. Setting these headers directly results in them largely being ignored, and warnings being raised at runtime. This filters out those headers from incoming Newman requests, allowing the underlying Spray client to handle those headers itself.

See https://github.com/spray/spray/blob/master/spray-http/src/main/scala/spray/http/HttpHeader.scala for more information.