`add_option()` and `update_option()` both call `get_option()` to compare the
value passed to the function with any existing value for the given option name.
When a `'default_option_'` filter is in place to change the default value of
an option, `add_option()` and `update_option()` ought to check against the
filtered value, rather than a hardcoded `false`, in order to determine whether
a prior value exists.
Props GregLone, tyxla.
Fixes#31047.
git-svn-id: https://develop.svn.wordpress.org/trunk@31473 602fd350-edb4-49c9-b593-d223f7449a82
94791cd8f2