[webpatser]

Closes #18

DocumentRoot::checkPreconditions() compared the timestamps the wrong way round: it answered 304 Not Modified when the file's mtime was newer than If-Modified-Since, i.e. exactly when the file had changed. Clients with a stale cache got an empty 304 instead of the new content, and clients with a fresh cache got a full 200.

The fix flips the comparison, so a 304 is sent only when mtime <= If-Modified-Since.

While in there I also made If-Modified-Since be ignored whenever If-None-Match is present, as RFC 9110 section 13.1.3 requires. A matching If-None-Match still returns 304 early; a non-matching one no longer falls through to the date check. It is a one-line change, but happy to split it out if you prefer. If-Unmodified-Since is untouched.

Tests (in test/DocumentRootTest.php):

• If-Modified-Since older than the mtime returns 200 with the body
• equal to the mtime returns 304
• newer than the mtime returns 304
• a non-matching If-None-Match plus a newer If-Modified-Since returns 200

testPreconditionNotModified asserted the buggy behaviour (an If-Modified-Since of 1970 giving 304), so it now sends the file's own mtime instead. The new tests fail on current 2.x and pass with this change; the full suite is green.

Heads up: the repo's CI currently fails at setup because of actions/cache@v2, as noted on #22, so the checks here will not show green for that reason alone.