Fix regression in Map deserializtion in Scala 2.13 when default typing is enabled.

This Map serializer is implemented in terms of the Java version and is
somewhat fragile, as discussed in FasterXML#643 / FasterXML#470.

This commit ensures that the wrapper class is a member class in both Scala and Java.

That results in this JSON as the serializer code chooses not to put
the inner class name in the type annotation.

```
{"m":["scala.collection.immutable.Map",{"one":"one","two":"two"}]}
```

Fixes FasterXML#643

Author: retronym

src/main/scala/com/fasterxml/jackson/module/scala/ser/MapSerializerModule.scala

@@ -8,20 +8,33 @@ import com.fasterxml.jackson.databind.ser.std.StdDelegatingSerializer
 import com.fasterxml.jackson.databind.util.StdConverter
 import com.fasterxml.jackson.module.scala.modifiers.MapTypeModifierModule
 
+import java.util
 import scala.collection.JavaConverters._
 import scala.collection.Map
 
 private class MapConverter(inputType: JavaType, config: SerializationConfig)
   extends StdConverter[Map[_,_],java.util.Map[_,_]]
 {
+  // Making this an inner class avoids deserializaion errors when polymorphic typing
+  // is enabled. In Scala 2.12 `delegate.asJava` happened to be an inner class but
+  // this implementation detail changed in 2.13.
+  //
+  // Tested in DefaultTypingMapDeserializerTest
+  private class MapWrapper[A, B](delegate: Map[A, B]) extends util.AbstractMap[A, B] {
+    private val wrapped = delegate.asJava
+
+    override def entrySet(): util.Set[util.Map.Entry[A, B]] = wrapped.entrySet()
+  }
+
   def convert(value: Map[_,_]): java.util.Map[_,_] = {
     val m = if (config.isEnabled(SerializationFeature.WRITE_NULL_MAP_VALUES)) {
       value
     } else {
       value.filter(_._2 != None)
     }
-    m.asJava
-  }
+    new MapWrapper(m)
+//    m.asJava
+  }``
 
 
   override def getInputType(factory: TypeFactory) = inputType

src/test/scala/com/fasterxml/jackson/module/scala/deser/DefaultTypingMapDeserializerTest.scala

@@ -0,0 +1,35 @@
+package com.fasterxml.jackson.module.scala.deser
+
+import com.fasterxml.jackson.databind.ObjectMapper
+import com.fasterxml.jackson.module.scala.DefaultScalaModule
+
+import scala.collection.immutable
+
+class DefaultTypingMapDeserializerTest extends DeserializerTest {
+
+  def module: DefaultScalaModule.type = DefaultScalaModule
+
+  override def newMapper: ObjectMapper = {
+    val mapper = super.newMapper
+    mapper.activateDefaultTyping(mapper.getPolymorphicTypeValidator)
+  }
+
+  "Scala Module" should "deserialize immutable Map when default typing enabled" in {
+    val map = HasMap(immutable.Map("one" -> "one", "two" -> "two"))
+
+    val mapper = newMapper
+
+    val json = mapper.writeValueAsString(map)
+    // Was failing in Scala 2.13+ with:
+    // > Could not resolve type id 'scala.collection.convert.JavaCollectionWrappers$MapWrapper' as a subtype of
+    // > `scala.collection.immutable.Map<java.lang.String,java.lang.String>`: Not a subtype
+    //
+    // prior the changing MapSerializerModule.scala to use an inner class for MapWrapper
+    val read = mapper.readValue(json, classOf[HasMap])
+
+    read shouldEqual map
+  }
+
+}
+
+case class HasMap(m: Map[String, String])