fix #240, ser of option with trait value type

Not sure if this is the best fix or not, but it at least
provides a simple inline test case to reproduce the issue
and passes all of the existing tests.

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

@@ -3,6 +3,7 @@ package module.scala
 package ser
 
 import java.lang.reflect.Type
+
 import com.fasterxml.jackson.annotation.JsonInclude
 import com.fasterxml.jackson.core.JsonGenerator
 import com.fasterxml.jackson.databind._
@@ -96,7 +97,10 @@ private class OptionSerializer(referredType: JavaType,
     ser = ser.orElse(valueSerializer).map(prov.handlePrimaryContextualization(_, prop)).asInstanceOf[Option[JsonSerializer[AnyRef]]]
     ser = Option(findConvertingContentSerializer(prov, prop, ser.orNull).asInstanceOf[JsonSerializer[AnyRef]])
     ser = ser match {
-      case None => if (hasContentTypeAnnotation(prov, prop)) {
+      case None => if (hasContentTypeAnnotation(prov, prop) && referredType.isContainerType) {
+        // In some cases `findValueSerializer` will return a BeanSerializer instance
+        // if the value type is a trait. This results in empty objects if the concrete
+        // value is a case class.
         Option(prov.findValueSerializer(referredType, prop)).filterNot(_.isInstanceOf[UnknownSerializer])
       } else None
       case Some(s) => Option(prov.handlePrimaryContextualization(s, prop).asInstanceOf[JsonSerializer[AnyRef]])

src/test/scala/com/fasterxml/jackson/module/scala/ser/OptionSerializerTest.scala

@@ -46,8 +46,20 @@ object OptionSerializerTest {
       _base = base
     }
   }
+
+  // For issue #240, for some reason it works fine when these classes are part
+  // of the object.
+  trait M1
+  case class F1(label: String) extends M1
+  case class C1(m: Option[M1])
 }
 
+// For issue #240, these need to be outside of the object to reproduce
+// the problem.
+trait M2
+case class F2(label: String) extends M2
+case class C2(m: Option[M2])
+
 @RunWith(classOf[JUnitRunner])
 class OptionSerializerTest extends SerializerTest {
   import OptionSerializerTest._
@@ -75,6 +87,19 @@ class OptionSerializerTest extends SerializerTest {
     serialize(noneOption) should be ("null")
   }
 
+  it should "serialize concrete type when using Option[Trait] (in object)" in {
+    // Additional test case for #240, for some reason this test case works fine.
+    // However, if the classes are moved outside of the object then it starts
+    // breaking.
+    serialize(C1(Some(F1("foo")))) should be ("""{"m":{"label":"foo"}}""")
+  }
+
+  it should "serialize concrete type when using Option[Trait]" in {
+    // See https://github.com/FasterXML/jackson-module-scala/issues/240 for more information.
+    // This test case reproduces the problem in the ticket.
+    serialize(C2(Some(F2("foo")))) should be ("""{"m":{"label":"foo"}}""")
+  }
+
   it should "serialize an Option[java.lang.Integer] when accessed on a class" in {
     case class Review(score: java.lang.Integer)
     val r1: Review = null