[cookies] fixes #309

The reason for this bug, is because we just changed the semantics of setHeader
to overwrite any existing identical header and the cookie behaviour was depending
on the addative behaviour

examples/Example/Program.fs

@@ -98,8 +98,13 @@ let app =
     path "/session"
       >>= statefulForSession // Session.State.CookieStateStore
       >>= context (fun x ->
-        match x |> HttpContext.state with
-        | None -> Redirection.FOUND "/session" // restarted server without keeping the key; set key manually?
+        match HttpContext.state x with
+        | None ->
+          // restarted server without keeping the key; set key manually?
+          let msg = "Server Key, Cookie Serialiser reset, or Cookie Data Corrupt, "
+                    + "if you refresh the browser page, you'll have gotten a new cookie."
+          OK msg
+
         | Some store ->
           match store.get "counter" with
           | Some y ->

src/Suave/Cookie.fs

@@ -114,7 +114,7 @@ module Cookie =
   let unsetCookie (cookieName : string) =
     let startEpoch = DateTimeOffset(1970, 1, 1, 0, 0, 1, TimeSpan.Zero) |> Some
     let stringValue = HttpCookie.toHeader { HttpCookie.mkKV cookieName "x" with expires = startEpoch }
-    Writers.setHeader "Set-Cookie" stringValue
+    Writers.addHeader "Set-Cookie" stringValue
 
   let setPair (httpCookie : HttpCookie) (clientCookie : HttpCookie) : WebPart =
     context (fun { runtime = { logger = logger } } ->