= can: expect connection close in HttpHostConnectionSlot after a response came back, fixes #510

Changes:
  * go into `closing` state if a response with `Connection: close` was received
  * also handle incoming requests in `closing` state properly, HttpHostConnectionSlot
    is thought to be idempotently able to receive new requests in every state
  * add spray.util.SimpleStash which provides functionality similar to
    `akka.actor.Stash` but without the ordering guarantee
= client: fix test that didn't seem to test anything because of small bugs

Fixes:
 * config settings under new namespace
 * path wasn't matched properly
 * too little max-retries to succeed against drop1of2