Prove error if "op" is boolean true

[outdooracorn]

The Postpone exception throwing commit reverts a fix from #60.

If the "op" field is boolean true, it is interpreted as an "add" operation instead of throwing an exception. This is due to true always matching the first switch case (Add::OP)

My bad for not covering it in a test!

This draft PR is to demonstrate the issue. How would you like to fix it?

[vearutop]

Oh, my bad, then I think it would be best to reapply your fix (move is_string check back to where it was, before the switch).

[vearutop]

(With more focus on Go these days, I'm losing my grip on PHP quirks 😅)

[outdooracorn]

No worries, happens to the best of us 😉

Closing this draft in preference of #63