Implemented a test harness for the itoa function (which is currently private to the http_response module).
Fixed handling of sufficiently large numbers.

itoa now uses all 32 characters of its buffer (one has been added to fit the null terminator in such cases), and takes unsigned rather than signed ints—since it's never used to represent negative numbers, and negative bases don't make any (at least practical) sense.

This fixes the previously-broken case tested by the harness from the previous commit.