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 https://github.com/denoland/deno/issues/17737
d4e5a295f2
