Fixes composer dependencies: Package phpunit/phpunit-mock-objects is abandoned
We cannot support v8 yet because of errors like:
Declaration of MailFunc::setUp() must be compatible with PHPUnit\Framework\TestCase::setUp(): void
It would require dropping PHP < 7.1 support.
The old version assumed `json_decode` would return an associative array.
While it can do so if an extra option is specified, the default
behaviour is to return an object. Therefore, a successful password
change resulted in an error while parsing the response.
The new code is accessing the response as object instead. The method
is now covered by a unit test.
