Allow uints(PdfUInteger) when calling ReadInt to support some encrypted file header reading.

This allows reading of uints by ReadInt and converts them to negative integers. This allows for reading some headers with other encryption options, even if we don't support that encryption type (useful if pdf still has read access without). Should not break most existing code unless code expected an exception for reading uints and was catching it.

Author: mitchcapper

PdfSharpCore/Pdf/PdfDictionary.cs

@@ -407,6 +407,9 @@ public int GetInteger(string key, bool create)
                 if (integerObject != null)
                     return integerObject.Value;
 
+                PdfUInteger uinteger = obj as PdfUInteger;
+                if (uinteger != null)
+                    return (int)uinteger.Value;
                 throw new InvalidCastException("GetInteger: Object is not an integer.");
             }