override writeReplace. fix serialize error

https://github.com/scala/scala/blob/v2.13.0-M5/src/library/scala/collection/Iterable.scala#L42-L46

Author: xuwei-k

shared/src/main/scala/scala/xml/MetaData.scala

@@ -225,4 +225,6 @@ abstract class MetaData
 
   final def remove(namespace: String, owner: Node, key: String): MetaData =
     remove(namespace, owner.scope, key)
+
+  protected[this] override def writeReplace(): AnyRef = this
 }

shared/src/main/scala/scala/xml/NodeSeq.scala

@@ -158,4 +158,6 @@ abstract class NodeSeq extends AbstractSeq[Node] with immutable.Seq[Node] with S
   override def toString(): String = theSeq.mkString
 
   def text: String = (this map (_.text)).mkString
+
+  protected[this] override def writeReplace(): AnyRef = this
 }