Custom header support

[Redsandro]

Hi there and welcome to my issue. I would like to take this opportunity and highlight the importance of custom header support on the server.

Every now and then there's a good reason. For example, Chromium's mixed content experiment has Chrome and Chromium blocking http media requests on https sites.

In order to disable this non-standard behavior, you need to explain to all your customers that they need to opt-out from this experiment, or simply send a mixed-content: noupgrade header with the html content of your Ember site.

Also #32.

Is this something that can be implemented?