Fix `X-Feide-`-headers not cleared by auth proxy.

`string.sub(k, 8)` in Lua extracts every character from the eight byte
onwards, and not the eight first bytes. Also, `x-feide` is 7 bytes,
not 8. The result is that we only delete headers named something like

This patch fixes this bug by correcting the `sub()`-call, and adding
the trailing `-` to the header name, bringing the length to 8
1 job for master in 2 minutes and 53 seconds (queued for 2 seconds)
Status Job ID Name Coverage
passed #7523