server doesn't take into account header change

## To reproduce
- Open a custom site in a server A (for example dev.massa-deweb.xyz)
- Add a header to this site via the cli metadata command like explained [here](https://docs.massa.net/docs/deweb/cli/metadatas#http-headers). Wait some time for the operation to be finalized
- If you reload the site, the header is not returned by the server A
- If you open the site in another server B (like the localhost for example) the header is returned as expected
- Now remove the header and wait some time for the operation to be finalized.
- The server B still return the header.

## How to
The error seems to be caused by a cache problem. If we manually remove the cache, the server take into account the header change.

Reproduce the header add/remove flow using the CLI metadata command and compare responses from server A and server B after finalization. Start by tracing the server cache handling for site headers; done means both servers reflect added headers and stop returning removed headers without manually clearing the cache.

Written by the indexing model from the issue text.