Handle deprecated `json_decode` from guzzle

[craigh]

if you look at the bottom of the output here: https://travis-ci.org/github/zikula/core/jobs/734982639

you can see several notices about deprecations of functions from guzzlehttp/guzzle

guzzle is required by league/oauth2-client

If we want to invest the effort then we should maybe entirely switch to using "\native_function" in general, like Symfony does it. This would improve performance because PHP wouldn't need to look for an equally-named method in the current namespace first.

[craigh]

I've since removed the Symfony-2-era deprecation detector (it was designed to help libraries migrate to Symfony 3). Since there is no longer a deprecation detector, there are no more deprecations 😉

Deprecations are now automatically logged to /var/log/dev.log.

In this log, there are no indications about json_decode so I am closing this ticket.