Set correct email message encoding

[4lexvav]

Description (*)

E-Mail subject not showing utf-8 characters.

I tried to investigate the problem and found that it happens because the utf-8 encoding is reset during sending an email. This happens here. The Zend\Mail\Message::fromString method recreates the Zend\Mail\Message object from the given string, but its default encoding is ASCII, so that the utf-8 encoding set while initializing the message gets reset.
In order to eliminate the issue the encoding have to be explicitly set again, like this: Message::fromString($this->message->getRawMessage())->setEncoding('utf-8').

Fixed Issues (if relevant)

1. Fixed issue E-Mail subject not showing utf-8 characters #19977: E-Mail subject not showing utf-8 characters

Manual testing scenarios (*)

1. Create an e-mail template containing the "Ő" letter in the subject. For example, based on the Newsletter Subscription Success email template.
2. Change Success Email Template at the system Newsletter settings.
3. Subscribe for newsletter.
4. View the email in your email client. In my case, macOS Mail.app.

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-cicd2]

All committers have signed the CLA.

[magento-engcom-team]

Hi @4lexvav. Thank you for your contribution
Here is some useful tips how you can test your changes using Magento test environment.
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[magento-engcom-team]

Hi @VladimirZaets, thank you for the review.
ENGCOM-3744 has been created to process this Pull Request

[ghost]

Hi @4lexvav, thank you for your contribution!
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[magento-engcom-team]

Hi @4lexvav. Thank you for your contribution.
We will aim to release these changes as part of 2.3.1.
Please check the release notes for final confirmation.

[magento-engcom-team]

Hi @4lexvav. Thank you for your contribution.
We will aim to release these changes as part of 2.3.1.
Please check the release notes for final confirmation.