If output is text/plain must call htmlToText conversion

Problem description

In some cases this module should convert HTML content to text but it doesn't.

Steps to reproduce:

1. Configure settings so that messages will be sent as Plain Text. Either a) enable respect_format or b) disable it and set output format to Plain Text.
2. Send a mail that is capable of generating markup. For example use the default contact form.
3. In the message include some special character - I used A & B <c

Expected: Message appears as typed
Actual: Message appears as HTML A &amp; B <c></c>

Proposed resolution

Drupal Core has the required code in PhpMail::format(). We need to call MailFormatHelper::htmlToText() if the output format is text/html and the input is markup. This code should go in SwiftMailer::massageMessageBody().