This was not caught in the previous test case, as the response body was smaller than the size of HEAD response.
This made nwritten < responseLen check in writeFixedResponse to fail, and not trigger op_flash_respond_async as a result.

When the response body is larger than the HEAD though, as in the updated test case (HEAD i 120 bytes, where our response is 300 bytes), it would think that we still have something to send, and effectively panic, as op_flash_respond already removed the request from the pool.

This change, makes the handleResponse function always calculate the number of bytes to transmit when HEAD request is encountered. Effectively ignoring Content-Length of the body, but still setting it correctly in the request header itself.

Fixes #17737