Headers are not sanitized

[StefanMich]

I am using silk in a project with a tastypie api, and when using apikey or basic auth these are sent as request header 'Authorization'. These headers are stored in silk which is security risk.

[StefanMich]

Implemented it with a hardcoded list, to be consistent with how bodies are masked.
It might be better to implement it as a setting so any header can be masked. I am happy to do that change if anyone finds that to be better

[nasirhjafri]

Added in release 4.0.0 (#384)

[chidg]

Great that you've done this @StefanMich and that @nasirhjafri has merged it, however is it possible to have it also merged into a maintained 3.x version? Silk 4.0 does not support Django <2.2, which means Django 1.11 which is currently still a supported LTS release doesn't have access to this important security fix.