validate() returns true with wrong top level elements

Hi,
I tried to run validate like this:
```
$spec = cebe\openapi\Reader::readFromJson($jsonstring);
$spec->resolveReferences(new cebe\openapi\ReferenceContext($spec, "/"));
$spec->validate();
```

on a OpenAPI 3 schema that has, for example the property "AAAAAcomponents" instead of "components", and the call returns true.
Is this correct?
Thanks,
Frank

Reproduce the example through Reader::readFromJson(), resolveReferences(), and validate() using an OpenAPI 3 document with a misspelled top-level property such as "AAAAAcomponents". Inspect how validation handles unknown top-level elements; done means the behavior is clarified and invalid documents are handled consistently.

索引モデルが issue の本文から書いたものです。