fix: let PHP handle basic auth. (#2142)
I noticed that PHP likes to handle and free basic auth parameters
internally (see
[here](https://github.com/php/php-src/blob/9f774e3a85d34f4aa1558613a9df7703ad3bb513/main/main.c#L2739)
and
[here](https://github.com/php/php-src/blob/9f774e3a85d34f4aa1558613a9df7703ad3bb513/main/SAPI.c#L514-L525)).
This PR changes it so the basic auth header is forwarded to PHP instead
of resolving it in go.
I suspect that this might fix some crashes in shutdown functions (like
#2121 and #1841) since it allows us freeing the `request_info` after
shutdown is finished. I haven't been able to reproduce these crashes yet
though.
