Support "or-ing" together open modes

[jmanm]

Resources

The official project web site:
http://pdfsharp.net/

The official peer-to-peer support forum:
http://forum.pdfsharp.net/

Reporting an Issue Here

Expected Behavior

No exception thrown when saving.

Actual Behavior

InvalidOperationException: "Cannot save a PDF document with no pages."

Steps to Reproduce the Behavior

using (var readStream = new MemoryStream())
using (var writeStream = new MemoryStream())
{
	var pdf = new PdfDocument();
	pdf.AddPage();
	pdf.Save(readStream);

	readStream.Seek(0, SeekOrigin.Begin);
	pdf = PdfReader.Open(readStream, PdfDocumentOpenMode.Import | PdfDocumentOpenMode.Modify);
	pdf.Save(writeStream);
}