Create instance of enum from same enum in constructor

[KartaviK]

What? - New feature
Will BC break - No

Problem:

Lets create MyEnum

<?php

final class MyEnum extends \MyCLabs\Enum\Enum
{
    public const MY_VALUE = 'test-value';
}

Trying to create instance:

<?php

$enum = new MyEnum(MyEnum::MY_VALUE); // perfectly created

And what if you create an enumeration from the same enumeration?

<?php

$sameEnum = new MyEnum($enum); // will catch semantically incorrect exception

Thrown exception:
Value 'test-value' is not part of MyEnum. But 'test-value' IS part of MyEnum.
This message looks like because __toString() method returns property value in string type if it is not override

This PR add simple logic to checking argument in constructor on instance of static and not throw exception if it is.

[mnapoli]

Thank you!