BaseMailer: strip <style> content from TextBody fix

If  ```$htmlLayout``` contains CSS <style> tags,
strip_tags here: https://github.com/yiisoft/yii2/blob/master/framework/mail/BaseMailer.php#L189
won't clean those CSS rules.

Actual html: 
```html
<html>
    <head>
        <style>
            .text-secondary {color: #93959A;}
            [...]
            [Other CSS rules]
            [...]
        </style>
    </head>
    <body>
        <div>Some content</div>
    </body>
</html>
```

Will look this way in TEXT version of Email: 
```
.text-secondary {color: #93959A;}
[...]
[Other CSS rules]
[...]
Some content
```

--------------

If TEXT version of Email differs drastically from visible content of HTML version,
it can be result that some penalty points will apply to such mails: 
http://wiki.apache.org/spamassassin/Rules/MPART_ALT_DIFF_COUNT


Since it's default behaviour for both yii2-app-advanced and yii2-app-basic (where stripped from tags html used for TextBody),
I believe people will stumble in this for sure, without even knowing that their emails can be penalized.


You can reproduce it by placing CSS styles in email view or layout
and test it here: http://www.mail-tester.com/ (SpamAssassin section).