(BC) fix embeds cant have 2 fields with same name

[SQKo]

Fixes #1142

Breaking changes for people using $embed->fields->offsetGet('name') or $embed->fields['name']

Tested using code from issue

Discord\Parts\Embed\Embed Object
(
    [title] =>
    [type] =>
    [description] =>
    [url] =>
    [timestamp] =>
    [color] => 16777215
    [footer] => Discord\Parts\Embed\Footer Object
        (
            [text] =>
            [icon_url] =>
            [proxy_icon_url] =>
        )

    [image] => Discord\Parts\Embed\Image Object
        (
            [url] =>
            [proxy_url] =>
            [height] =>
            [width] =>
        )

    [thumbnail] => Discord\Parts\Embed\Image Object
        (
            [url] =>
            [proxy_url] =>
            [height] =>
            [width] =>
        )

    [video] => Discord\Parts\Embed\Video Object
        (
            [url] =>
            [proxy_url] =>
            [height] =>
            [width] =>
        )

    [provider] =>
    [author] => Discord\Parts\Embed\Author Object
        (
            [name] =>
            [url] =>
            [icon_url] =>
            [proxy_icon_url] =>
        )

    [fields] => Discord\Helpers\Collection Object
        (
            [0] => Discord\Parts\Embed\Field Object
                (
                    [name] => ​
                    [value] => ​
                    [inline] =>
                )

            [1] => Discord\Parts\Embed\Field Object
                (
                    [name] => ​
                    [value] => ​
                    [inline] =>
                )

        )

)