Add support for custom headers

[fool]

- Do you want to request a feature or report a bug?

feature

- What is the current behavior?

netlify custom headers (cf https://www.netlify.com/docs/headers-and-basic-auth/#custom-headers) are not supported

- If the current behavior is a bug, please provide the steps to reproduce.

attempt to use a custom header from a dev instance, do not see the expected value

- What is the expected behavior?

attempting to use a path covered by a header from a dev instance should return contents with that custom header

- Please mention your node.js, and operating system version.

all

[Illyism]

Would be nice to test your headers setup, such as Content-Security-Policy before deploy

[overlordofmu]

Tracking:

https://app.intercom.io/a/apps/q245f50x/inbox/inbox/1479299/conversations/22260577749

[kennedyrose]

Would absolutely love to see this. We have to pass headers to hit up our internal APIs and without this it's impossible to test before deploying.

[MichalStaruch]

Honoring headers like CSP defined in _headers file would be a great feature to see in netlify dev. Without that development pipeline loses a lot of consistency. Site that looks good on localhost may be a total mess when deployed on production (strict CSP won't allow inline JS and CSS to run, for example).