Support for access response headers

Hi all

*As a client side developer, for the response of the current page, I would
like to be able to access the headers that the browser received. Currently
I can do this for any ajax request that I make, but not for the root
response itself.*

As far as I can tell, access headers has been loosely discussed in the
past, but not as a main topic. Getting access to these headers would be
hugely helpful and facilitate a number of use cases that are currently very
difficult to handle.

*My primary use cases* is to be able to set a response header which
represents the ID of the request. Then as various client profiling events
happens, I can notify the server which request they correlate to.
Currently, I have to try and set this ID as a `data` attribute on the
script or inject a meta tag in page on the server. Modifying the content of
the page becomes difficult when trying trying to create modules that are
cross cutting and shouldn't require the user to modify the content of their
page.

Would be interested in peoples thoughts and if other people would use this
info as well.

Cheers
Anthony