Change AddHeader method to append header value if one already exists

Use Owin AppendValues helper method to append a header value if a value already exists. It's needed if another owin middleware has already set the WWW-Authenticate header.
Merge pull request #55 from ShannonKasperNoesis/patch-1

Change AddHeader method to append header value if one already exists
Merge remote-tracking branch 'source/master'