in src/main/scala/com/gu/fastly/api/FastlyApiClient.scala [92:95]
def purgeKey(key: String, extraHeaders: Map[String, String] = Map.empty): Future[Response] = {
val apiUrl = s"$fastlyApiUrl/service/$serviceId/purge/$key"
AsyncHttpExecutor.execute(apiUrl, POST, headers = Map("Fastly-Key" -> apiKey) ++ extraHeaders)
}