Most (all?) implementations of `write(ArrayPtr<const ArrayPtr<const byte>>)`, if the outer array contains only one inner array, do not use the outer array again after the initial call returns (as opposed to the promise resolving). But, this is not a safe assumption and http-test.c++ should not be relying on it.

(I found this when I tried forcing all writes to complete asynchronously to check if it resulted in any bugs. This is all I found.)