Show empty response body size in access log when response body MUST be empty

[clue]

This changeset ensures we always log an empty response body size in the access log when the response body MUST be empty, such as for HEAD requests, 204 No Content and 304 Not Modified.

2021-01-29 12:22:01.717 - "GET /users HTTP/1.1" 304 0 0.000

Builds on top of #45, #46, #47, #48, #49
Refs reactphp/http#429