v1.3.1: Make "Refresh now" update the cache the site actually reads

Make "Refresh now" update the cache the site actually reads

The shortcode resolves the configured default URL itself before calling
WCAL_Feed::get_events(), but is_override() treated any non-empty URL as a
second calendar deserving its own cache keys. So the public page read
wcal_events_, while the settings page's "Refresh now" — which passes
no URL — primed plain wcal_events. A manual refresh reported the freshly
fetched event count while writing to a cache nothing ever read, leaving new
events invisible until the front end's own copy expired hours later.

is_override() now compares against the configured default instead of just
testing for emptiness, so resolving the default URL before handing it over
stays on the shared keys. Genuine per-shortcode overrides are unaffected.

Co-Authored-By: Claude Opus 5 noreply@anthropic.com