Fix html body mail on SwiftMailCollector

[parallels999]

• Issue introduced on Capture HTML body for mails #617

After #617, SwiftMailCollector stop showing messages body preview,
Now html array key is needed

php-debugbar/src/DebugBar/Bridge/Symfony/SymfonyMailCollector.php

Lines 65 to 66 in 0b40770

	$mail['html'] = $message->getHtmlBody();
	$mail['body'] = $message->getTextBody();

[barryvdh]

Hmm maybe we should check if the key exists

[barryvdh]

Is the body actually html here? If so, the other body can be null or the text version

[barryvdh]

I think this was already fixed with #618 though, can you confirm?

[barryvdh]

If it's html, we need to add it to html, otherwise to the body; #620