Account for widget callbacks being array objects

[westonruter]

Fixes #64.

See also support forum topic: https://wordpress.org/support/topic/php-recoverable-fatal-error-when-switching-amp-mode/

This implements a fix I proposed in another support topic: https://wordpress.org/support/topic/standard-template-mode-internal-error/#post-12547053

This fixes compatibility with the AMP plugin which wraps all the registered widget callbacks with an object that implements ArrayAccess.