[Backport] Fix meta title property

[slackerzz]

Original Pull Request

magento#11368

Description

If, inside a controller you try to set metadata title with:

$resultPage->getConfig()->setMetadata('title', 'some meta title');

you will get an error on frontend:

Recoverable Error: Object of class Magento\Framework\View\Page\Title could not be converted to string...

becouse processMetadataContent will return an istance of Magento\Framework\View\Page\Title instead of a string.

Fixed Issues (if relevant)

1. Unable to render page when 'meta title' page config param is set magento/magento2#2956: Unable to render page when 'meta title' page config param is set

Manual testing scenarios

Scenario 1

1. Create a controller
2. Inside the execute function add:

$resultPage = $this->resultPageFactory->create();
$resultPage->getConfig()->getTitle->set('my title');
$resultPage->getConfig()->setMetadata('title', 'metatitle');
return $resultPage;

3. As result you will have in your page:

<title>my title</title>
<meta name="title" content="metatitle"/>

Scenario 2

1. Create a controller
2. Inside the execute function add:

$resultPage = $this->resultPageFactory->create();
$resultPage->getConfig()->getTitle->set('my title');
return $resultPage;

3. As result you will have in your page:

<title>my title</title>
<meta name="title" content="my title"/>

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)