In follow-up to the related change, mention the new
cors_expose_headers option (and other proxy-server.conf
options) in the CORS doc.
Add a test for the cors options being loaded into the
proxy server.
Improve CORS comments in docs.
Change-Id: I647d8f9e9cbd98de05443638628414b1e87d1a76
Related-Change: I5ca90a052f27c98a514a96ee2299bfa1b6d46334