Use the exsisting link collection when properties are links.

This seems to be the expected behaviour, as every other link uses the 'relation' property of the link. 

Allows resources to contain an embedded resource and a link with the same name:
```
$resource['account'] = $account; //object
$link = new Link('account'); //this is the name
$link->setRoute('accounts');
$link->setRouteParams(['id' => $account->getId()]);
$resource['account_link] = $link; //this key doesn't really matter
```

Output:
```
{
    "id": "1234",
    "_embedded": {
        "account": {
            "id": "5678",
            "name": "Account Name",
            "_links": {
                "self": {
                    "href": "http://localhost/accounts/5678"
                }
            }
        }
    },
    "_links": {
        "self": {
            "href": "http://localhost/users/1234"
        },
        "account": {
            "href": "http://localhost/accounts/5678"
        }
    }
}
```


Also enables arrays of links:
```
$link1 = new Link('friends');
$link1->setUrl('users/1234');
$link1->setProps(['name' => 'Tim']);

$link2 = new Link('friends');
$link2->setUrl('users/5678');
$link2->setProps(['name' => 'Kate']);

$resource[] = $link1;
$resource[] = $link2;
```

Output:
```
{
    "links": {
        "friends": [
            {
                "name": "Tim",
                "href": "http://localhost/users/1234"
            },
            {
                "name": "Kate",
                "href": "http://localhost/users/5678"
            }
        ]
    }
}
```