Allow CDN broker to forward all headers

[afeld]

@sharms commented on Fri Dec 16 2016

In order to enable applications to work as expected, customers should be able to configure which original headers should be forwarded.

Notes: Otherwise login pages etc may be cached instead of working properly.

Acceptance Criteria:

• [x] Method of setting headers to be forwarded when using CDN broker
• [ ] Instructions are on docs.cloud.gov

[afeld]

Potentially part of #50.

[46bit]

This seems resolved by #90.

[afeld]

Awesome, thanks!