[spray-client] HttpConduit close leaves connections open and throws exception in MainActor when stopped

[alexdoth]

If the Http session is closed in HttpConduit, the mainActor will be stopped, but the outstanding futures waiting to be completed still have a reference to the stopped actor. When the futures complete an exception is raised in the actor thread, and printed to standard error.

val conduit = new HttpConduit("github.com")
conduit.sendReceive(HttpRequest())
conduit.close() 
// Prints akka.actor not started error to standard error

The test case is in HttpConduitSpec:handleClose