Fix /Contents handling for signatures in encrypted documents

We need to explicitly mark the /Contents string as non-encrypted,
otherwise it gets decrypted automatically to gibberish, if a document
is encrypted.

Author: Eswcvlad

src/main/java/com/itextpdf/rups/model/TreeNodeFactory.java

@@ -269,6 +269,9 @@ private static boolean expandSigContentsNode(PdfObjectTreeNode node) {
          * themes, but it seems like a reasonable default to use.
          */
         final PdfString nodeObject = (PdfString) node.getPdfObject();
+        // This is important, otherwise /Contents value in encrypted documents
+        // will be broken...
+        nodeObject.markAsUnencryptedObject();
         final AbstractAsn1TreeNode asn1 = Asn1TreeNodeFactory.fromPrimitive(nodeObject.getValueBytes());
         if (asn1 != null) {
             /*