Allow passing prepared objects instead of array definition

[SilverFire]

Before this pull request I was able to write only

new Server([ 
    'url' => "{protocol}://$host/",
    'variables' => [
        'protocol' => [
            'enum' => ['http', 'https'],
            'default' => 'https', 
        ],
    ],
])

It's could be enough, but when it comes to more complex code, I would like to write something like that to be more verbose on what is going on inside my object:

new Server([ 
    'url' => "{protocol}://$host/",
    'variables' => [
        'protocol' => new ServerVariable([
            'enum' => ['http', 'https'],
            'default' => 'https',
        ]),
    ],
])

This pull request enhances library so it respects the passed object.

[SilverFire]

Hi, Carsten! I've added a test that covers SpecBaseObject, Paths and PathItem behavior at once.

[cebe]

Merged, thank you!