Set the response reason-phrase (deprecated)

void
reason(
    string_view s);

This function sets a custom reason-phrase to a copy of the string passed in. Normally it is not necessary to set the reason phrase on an outgoing response object; the implementation will automatically use the standard reason text for the corresponding status code. To clear a previously set custom phrase, pass an empty string. This will restore the default standard reason text based on the status code used when serializing. The reason-phrase is obsolete as of rfc7230.

This function is only available when isRequest == false.