issues/339: Send Allow http header when we respond with http 405 status code.

[komuw]

• This is mandated by the spec

• Fixes: https://github.com/komuw/ong/issues/339

• https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Allow

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.02% :tada:

Comparison is base (c20db72) 74.96% compared to head (ede9964) 74.98%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #345 +/- ## ========================================== + Coverage 74.96% 74.98% +0.02% ========================================== Files 44 44 Lines 4965 4970 +5 ========================================== + Hits 3722 3727 +5 Misses 995 995 Partials 248 248 ``` | [Files Changed](https://app.codecov.io/gh/komuw/ong/pull/345?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Komu+Wairagu) | Coverage Δ | | |---|---|---| | [middleware/middleware.go](https://app.codecov.io/gh/komuw/ong/pull/345?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Komu+Wairagu#diff-bWlkZGxld2FyZS9taWRkbGV3YXJlLmdv) | `99.11% <100.00%> (+0.01%)` | :arrow_up: |

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