DotExporter exports outdated version of the graph

[stheid]

 * JGraphT version:                                'org.jgrapht:jgrapht-io:1.5.1'
 * Java version (java -version)/platform:  java-11-openjdk (linux)

Issue
When using Nodes that contain classes, the dotexporter seems to export a cached version of the graph instead of properly evaluating all classes toString. This causes wrong outputs.

Steps to reproduce (small coding example)

import org.jgrapht.graph.DefaultDirectedGraph
import org.jgrapht.graph.DefaultEdge
import org.jgrapht.nio.dot.DOTExporter
import java.io.File


class Node(val wrapper: StringWrap)
class StringWrap(var value: String)

fun main() {
    val graph = DefaultDirectedGraph<Node, DefaultEdge>(DefaultEdge::class.java)
    val node1 = Node(StringWrap("old"))
    graph.addVertex(node1)

    val exporter = DOTExporter<Node, DefaultEdge> { """"${it.wrapper.value}"""" }
    exporter.exportGraph(graph, File("old.dot").bufferedWriter()) // <- commenting out this line will eliviate the bug
    node1.wrapper.value = "new"
    exporter.exportGraph(graph, File("new.dot").bufferedWriter())
}

Than new.dot contains the old data

strict digraph G {
  "old";
}

Expected behaviour

new.dot containing the updated data

strict digraph G {
  "new";
}

Other information
The problem is actually caused by the first export call. This somehow caches information inside the exporter. When commenting out the first export call the result is correct.

Also it is importaint that the data is contained inside a class. It seems that it checks if the class reference has changed. changing the updating code to node1.wrapper = new StringWrap("new") will work!.