Fix handling of tiny invalid responses (#422)

Fix handling of tiny invalid responses (#422)

Squid converted any invalid response shorter than 4 bytes into an
invalid "HTTP/1.1 0 Init" response (with those received characters and a
CRLFCRLF suffix as a body). In some cases (e.g., with ICAP RESPMOD), the
resulting body was not sent to the client at all.

Now Squid handles such responses the same way it handles any non-HTTP/1
(and non-ICY) response, converting it into a valid HTTP/200 response
with an X-Transformed-From:HTTP/0.9 header and received bytes as
a message body.