wp_get_nocache_headers should always return an array

[filosofo]

Apparently  sometimes plugins use its filter and don't return an array.

WP should instead cast filtered results to array, so it always returns an array.

It's up to plugins to return a correctly formatted value. Why cast the return value of this one particular filter? Is it any different to the others?

The net number of casts to array is unchanged, just moved to make it more reliable.

In general, I think it's better to try to handle plugins' runtime errors (like this) gracefully, as opposed to say more "compile"-time errors (including the wrong file or using the wrong protocol, etc.).

That's because the latter kind of error is more likely to appear consistently during development and therefore get corrected. But the former might occur only in particular conditions.

The wontfix brought me to look at the patch. Looks solid. Definitely a better place for the cast.

(In [16093]) Move an array cast further down the stack. props filosofo, fixes #13965.