Send a header to the back end identifying that Shunter has proxied the request

[rowanmanning]

Currently Shunter sends a timestamp in the shunter request parameter, it would be good to also add a header to the proxied request to identify that Shunter has proxied the request – we've come across a scenario where sometimes the back end is accessed directly, and needs to change behaviour based on whether the request came through Shunter.

I'm thinking we could maybe expose the Shunter version or something as well, which might be useful for debugging. Something like:

X-Shunter: 3.0.2

[moddular]

Sounds good to me.

[andrewmee]

This has been added with https://github.com/nature/shunter/releases/tag/3.1.0