[PropertyAccess] array_keys does not work for ArrayAccess objects

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

The array_keys() function does not work on objects implementing \ArrayAccess. Skip listing the known properties for such objects.