Include default match statement to CaseClassCodec macro

Author: jakehschwartz

bson-scala/src/main/scala/org/mongodb/scala/bson/codecs/macrocodecs/CaseClassCodec.scala

@@ -350,7 +350,7 @@ private[codecs] object CaseClassCodec {
                   val instanceValue = value.asInstanceOf[${classType}]
                   ..${writeClassValues(fields, ignoredFields(classType))}"""
         }.toSeq
-      }
+      } :+ cq"""_ => throw new BsonInvalidOperationException("Unexpected class type: " + className)"""
       q"""
         writer.writeStartDocument()
         this.writeClassFieldName(writer, className, encoderContext)