change serialization to strictly rely on subtyping

- when encountering new message type, check all bindings which map apply
- if multiple are found, choose the most specific one if that exists or
  verify that all mappings yield the same serializer
- in case of remaining ambiguity, throw exception
- also add special handling for “none” serializer mapping: turn off a
  default

akka-actor/src/main/scala/akka/serialization/Serialization.scala

@@ -125,17 +125,23 @@ class Serialization(val system: ExtendedActorSystem) extends Extension {
   def serializerFor(clazz: Class[_]): Serializer =
     serializerMap.get(clazz) match {
       case null ⇒
-        val ser = bindings.collectFirst {
-          case (c, s) if c.isAssignableFrom(clazz) ⇒ s
-        } getOrElse (throw new NotSerializableException(
-          "No configured serialization-bindings for class [%s]" format clazz.getName))
+        // bindings are ordered from most specific to least specific
+        def unique(cs: Seq[Class[_]], ser: Set[Serializer]): Boolean = (cs forall (_ isAssignableFrom cs(0))) || ser.size == 1
 
-        // memorize for performance
-        serializerMap.putIfAbsent(clazz, ser) match {
-          case null ⇒
-            log.debug("Using serializer[{}] for message [{}]", ser.getClass.getName, clazz.getName)
-            ser
-          case some ⇒ some
+        val possible = bindings filter { _._1 isAssignableFrom clazz }
+        possible.size match {
+          case 0 ⇒
+            throw new NotSerializableException("No configured serialization-bindings for class [%s]" format clazz.getName)
+          case x if x == 1 || unique(possible map (_._1), possible.map(_._2)(scala.collection.breakOut)) ⇒
+            val ser = possible(0)._2
+            serializerMap.putIfAbsent(clazz, ser) match {
+              case null ⇒
+                log.debug("Using serializer[{}] for message [{}]", ser.getClass.getName, clazz.getName)
+                ser
+              case some ⇒ some
+            }
+          case _ ⇒
+            throw new NotSerializableException("Multiple serializers found for " + clazz + ": " + possible)
         }
       case ser ⇒ ser
     }
@@ -160,7 +166,7 @@ class Serialization(val system: ExtendedActorSystem) extends Extension {
    *  It is primarily ordered by the most specific classes first, and secondly in the configured order.
    */
   private[akka] val bindings: Seq[ClassSerializer] = {
-    val configuredBindings = for ((k: String, v: String) ← settings.SerializationBindings) yield {
+    val configuredBindings = for ((k: String, v: String) ← settings.SerializationBindings if v != "none") yield {
       val c = ReflectiveAccess.getClassFor(k, system.internalClassLoader).fold(throw _, identity[Class[_]])
       (c, serializers(v))
     }