Add handler for CORS "actual requests"

Fix for bug 1095130

* Added a wrapper function around public methods to handle
  CORS actual requests. These requests need to return some
  extra headers to be valid responses to a CORS request.
  Access-Control-Expose-Headers and Access-Control-Allow-Origin.

* Added support for the CORS header Access-Control-Expose-Headers.

* Some refactoring of the OPTIONS method so the
  "is_origin_allowed" logic can be reused.

* Added a little extra detail to the CORS documentation.

DocImpact

Change-Id: I68538e472a900775427f21a8a59e738a83dcc8bc
Merge "Add handler for CORS "actual requests""