Replace usage of internal java.xml class with public abstraction (#1098)

This commit fixes problem of using DocumentBuilderFactoryImpl from com.sun.org.apache.xerces.internal.jaxp package. Problem arises as this package is not public (not exported) since JDK 9. Solution is to replace use of this class with DocumentBuilderFactory from exported package java.xml.parsers.

src/main/java/com/github/tomakehurst/wiremock/common/Xml.java

@@ -15,7 +15,6 @@
  */
 package com.github.tomakehurst.wiremock.common;
 
-import com.sun.org.apache.xerces.internal.jaxp.DocumentBuilderFactoryImpl;
 import org.custommonkey.xmlunit.XMLUnit;
 import org.w3c.dom.Document;
 import org.w3c.dom.Node;
@@ -150,14 +149,40 @@ public static DocumentBuilderFactory newDocumentBuilderFactory() {
         }
     }
 
-    public static class SkipResolvingEntitiesDocumentBuilderFactory extends DocumentBuilderFactoryImpl {
+    public static class SkipResolvingEntitiesDocumentBuilderFactory extends DocumentBuilderFactory {
+        private final DocumentBuilderFactory documentBuilderFactory;
+
+        public SkipResolvingEntitiesDocumentBuilderFactory() {
+            documentBuilderFactory = DocumentBuilderFactory.newInstance();
+        }
+
         @Override
         public DocumentBuilder newDocumentBuilder() throws ParserConfigurationException {
-            DocumentBuilder documentBuilder = super.newDocumentBuilder();
+            DocumentBuilder documentBuilder = documentBuilderFactory.newDocumentBuilder();
             documentBuilder.setEntityResolver(new SkipResolvingEntitiesDocumentBuilderFactory.ResolveToEmptyString());
             return documentBuilder;
         }
 
+        @Override
+        public void setAttribute(String name, Object value) throws IllegalArgumentException {
+            documentBuilderFactory.setAttribute(name, value);
+        }
+
+        @Override
+        public Object getAttribute(String name) throws IllegalArgumentException {
+            return documentBuilderFactory.getAttribute(name);
+        }
+
+        @Override
+        public void setFeature(String name, boolean value) throws ParserConfigurationException {
+            documentBuilderFactory.setFeature(name, value);
+        }
+
+        @Override
+        public boolean getFeature(String name) throws ParserConfigurationException {
+            return documentBuilderFactory.getFeature(name);
+        }
+
         private static class ResolveToEmptyString implements EntityResolver {
             @Override
             public InputSource resolveEntity(String publicId, String systemId) throws SAXException, IOException {