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-tests/src/test/scala/akka/serialization/SerializeSpec.scala

@@ -179,14 +179,10 @@ class SerializeSpec extends AkkaSpec(SerializeSpec.config) {
       ser.serializerFor(classOf[ExtendedPlainMessage]).getClass must be(classOf[TestSerializer])
     }
 
-    "resolve serializer for message with several bindings" in {
-      ser.serializerFor(classOf[Both]).getClass must be(classOf[TestSerializer])
-    }
-
-    "resolve serializer in the order of the bindings" in {
-      ser.serializerFor(classOf[A]).getClass must be(classOf[JavaSerializer])
-      ser.serializerFor(classOf[B]).getClass must be(classOf[TestSerializer])
-      ser.serializerFor(classOf[C]).getClass must be(classOf[JavaSerializer])
+    "throw exception for message with several bindings" in {
+      intercept[java.io.NotSerializableException] {
+        ser.serializerFor(classOf[Both])
+      }
     }
 
     "resolve serializer in the order of most specific binding first" in {