title: NotSerializableException for `Map[String, String]` alias
body: <p>I'm trying to send an object to a remote actor and I got this exception:</p>
  
  <pre><code>ERROR akka.remote.EndpointWriter - Transient association error (association remains live)
  java.io.NotSerializableException: scala.collection.immutable.MapLike$$anon$2
  </code></pre>
  
  <p>The object being serialized is a case class:</p>
  
  <pre><code>case class LocationReport(idn: String, report: String, timestamp: Option[String], location: Attr, status: Attr, alarms: Attr, network: Attr, sensors: Attr) extends Message(idn) {
  
    val ts = timestamp getOrElse location("fix_timestamp")
  
    def json =
      (report -&gt;
        ("TIME" -&gt; ts) ~
        ("location" -&gt; location) ~
        ("alarms" -&gt; alarms) ~
        ("network" -&gt; network) ~
        ("sensors" -&gt; ((status ++ sensors) + ("CUSTOMCLOCK" -&gt; Report.decodeTimestamp(ts)))))
  }
  </code></pre>
  
  <p>And <code>Attr</code> is a type re-definition:</p>
  
  <pre><code>type Attr = Map[String, String]
  </code></pre>
  
  <p>The <code>Message</code> class is pretty simple:</p>
  
  <pre><code>abstract class Message(idn: String) {
    def topic = idn
    def json(): JValue
  }
  </code></pre>
  
  <p>I'm wondering if the type alias/redefinition is confusing the serializer. I think I'm using ProtoBuf serialization, but I do see <code>JavaSerializer</code> in the stacktrace.</p>
  
  <p><strong>More Debugging Info</strong></p>
  
  <p>I newed up a JavaSerializer and individually serialized each of the Maps. Only one (<code>alarms</code>) fails to serialize. Here's the toString of each of them:</p>
  
  <p>This one failed:</p>
  
  <pre><code>alarms = Map(LOWBATTERY -&gt; 1373623446000)
  </code></pre>
  
  <p>These succeeded:</p>
  
  <pre><code>location = Map(a_value -&gt; 6, latitude -&gt; 37.63473, p_value -&gt; 4, longitude -&gt; -97.41459, fix_timestamp -&gt; 3F0AE7FF, status -&gt; OK, fix_type -&gt; MSBL, CUSTOMCLOCK -&gt; 1373644159000)
  network = Map(SID -&gt; 1271, RSSI -&gt; 85)
  sensors = Map(HUMIDITY -&gt; -999, PRESSURE -&gt; -999, LIGHT -&gt; -999  9:52 AM)
  status = Map(TEMPERATURE_F -&gt; 923, CYCLE -&gt; 4, TEMPERATURE1_C -&gt; 335, CAP_REMAINING -&gt; 560, VOLTAGE -&gt; 3691, CAP_FULL -&gt; 3897)
  </code></pre>