[Backport 2.3 -> 2.2] Fix meta title property & Updated unit test

[gelanivishal]

Original Pull Request

#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 #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)

[magento-engcom-team]

Hi @vgelani. Thank you for your contribution
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[orlangur]

@vgelani please do not rewrite authors of original commits during porting. Feel free to do just a one commit with git commit -c 32e2e4de5921ecb7b13722955298e886ec2b2d59, just that @slackerzz should be properly credited as author and you as author of port will be mentioned as committer.

[gelanivishal]

@orlangur Yeah, I got it. I'll take care in next PR.

[orlangur]

@vgelani, thanks for your endeavors, @slackerzz will take care of this PR porting. He noticed some changes were missed which resulted in failing unit test.