Objects with __invoke method can not be used in custom methods

Not something I need, but still a bug IMO:

``` php
$callable = new class {
    public function __invoke() {}
};

Phony::mock(
    null,
    ['method' => $callable]
);
```

Probably just needs some logic to construct a Reflector from `[$value, '__invoke']`, as the failure is:

```
TypeError: ReflectionFunction::__construct() expects parameter 1 to be string, object given
```
