properly add signature field to acroform array

so that PDF readers can properly detect the signature

Author: julienrffr

src/foundation/src/PDFsharp/src/PdfSharp/Pdf.AcroForms/PdfAcroField.cs

@@ -267,7 +267,7 @@ public PdfAcroFieldCollection Fields
         /// </summary>
         public sealed class PdfAcroFieldCollection : PdfArray
         {
-            PdfAcroFieldCollection(PdfArray array)
+            internal PdfAcroFieldCollection(PdfArray array)
                 : base(array)
             { }
 

src/foundation/src/PDFsharp/src/PdfSharp/Pdf.Signatures/PdfSignatureHandler.cs

@@ -1,4 +1,4 @@
-// PDFsharp - A .NET library for processing PDF
+// PDFsharp - A .NET library for processing PDF
 // See the LICENSE file in the solution root for more information.
 
 using PdfSharp.Drawing;
@@ -129,7 +129,7 @@ private void AddSignatureComponents(object sender, EventArgs e)
             }
 
             if (catalog.AcroForm.Elements.GetValue(PdfAcroForm.Keys.Fields) == null)
-                catalog.AcroForm.Elements.SetValue(PdfAcroForm.Keys.Fields, new PdfArray());
+                catalog.AcroForm.Elements.SetValue(PdfAcroForm.Keys.Fields, new PdfAcroField.PdfAcroFieldCollection(new PdfArray()));
             catalog.AcroForm.Fields.Elements.Add(signatureField);
         }