Reference nits

There is one small change which need to be made to the references:

Accidentally RFC 2069 is not present in the references, despite being cited 
in Section 15.1.  So, the following reference needs to be added:

[Franks et al., 1997] J. Franks, P. Hallam-Baker, J. Hostetler, P. Leach, 
A. Luotonen, E. Sink, L. Stewart, "An Extension to HTTP: Digest Access 
Authentication." RFC 2069. Northwestern University, CERN, Spyglass, 
Microsoft, Netscape, Open Market. January, 1997.

- Jim