Takes back what a response can no longer say of itself once its body
is going to be another: that ranges of it can be asked for, and that
its ETag names these bytes.
A list of strings. One string on its own is a list of one: that is how
a single value reads in KDL (ip_list "1.2.3.4"), and it used to count
as no list at all, which left an allow or deny list empty without a
word.