[pocketarc]

Several minor tweaks to preserve backward compatibility, along with documentation updates and new tests for the merged fixes.

• A missing cache_path setting no longer makes the page and file caches use the filesystem root.
• The file cache now works when cache_path has no trailing slash.
• CAPTCHA image names match the returned time again, as documented.
• Redis counters keep working with keys created before 3.2-dev.
• Redis session save paths accept uppercase schemes such as TLS://.
• Watermarked WebP images keep their transparency.
• set_cookie() with an array value fails loudly again, instead of setting a cookie called Array.