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
`???????X-Feide`.

This patch fixes this bug by correcting the `sub()`-call, and adding
the trailing `-` to the header name, bringing the length to 8
characters.
1 job for v0.3 in 16 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Deploy
passed #7528
stable

00:00:16