Restore PURGE miss replies to be "404 Not Found", not "0 Init" (#586)

Restore PURGE miss replies to be "404 Not Found", not "0 Init" (#586)

Since commit 6956579, PURGE requests resulted in invalid HTTP responses
with zero status code when Store lacked both GET and HEAD entries with
the requested URI.

Also adjusted Http::StatusLine packing code to avoid generating similar
invalid responses in the future (and to attract developer attention to
their presence in the code logic with a BUG message).

This is a Measurement Factory project.