Subtle code errors

[ScottMac]

While doing some tests of a new tool we came across some code errors, one is a comparison that should be an assignment.

The others are calling methods with more parameters than they have in the caching layer.

See [6539], looks like that's a deprecated argument for WP_Object_Cache::get() from when persistent cache was pulled.

If you upload patches with a ".diff" or ".patch" extension, Trac will show them much better.

Also, not sure if it's worth fixing, but s/existant/existent/ would correct a misspelling in a class property.

(In [13054]) Fix typos. Fix assignment. Remove passing of non-existent arg. Props ScottMac. fixes #12193