Mostly revert "Verify parameter for MapCacheLRU::has() can be passed to array_key_exi...

This broke a few things, and the debug line was basically pointless.

Instead, continue to only throw an exception if $text is an object, but only
warn if it's otherwise not a string.

This reverts commit 372ded2fea436bf0e61318991f903d61bde26366.

Change-Id: I060da9191cdbd00c4873caba875bfb77c917bcd7