Add 'X-Requested-With' header to iframe

== Proposal ==
Requests sent to the embed source from a iframe should contain the the
header X-Requested-With with the value iframe.

X-Requested-With: IFrame

== Rationale ==
This allows the server-side to know if the resource was requested manually
by a user or it was embedded in a page through using the iframe element.

The server may chose to deny requests which are expected to be iframed but
are not requested with the expected header.