[Forwardport] magento/magento2#13551 Fix json encoded attribute backend type to not encode attribute value multiple times

[nmalevanec]

Original Pull Request

#13551 (created by @tkotosz)

Included Commits

1. 85f36cd
2. 3d10764

Manual testing scenarios

1. Create custom product attribute with backend model = "\Magento\Eav\Model\Entity\Attribute\Backend\JsonEncoded" ex:

    /**
     * Installs data for a module
     *
     * @param ModuleDataSetupInterface $setup
     * @param ModuleContextInterface $context
     * @return void
     */
    public function install(ModuleDataSetupInterface $setup, ModuleContextInterface $context)
    {
        $setup->startSetup();
        /** @var EavSetup $eavSetup */
        $eavSetup = ObjectManager::getInstance()->get(EavSetupFactory::class)->create();
        $eavSetup->addAttribute(
            4,
            'test_json_encoded_attribute',
            [
                'type' => 'text',
                'label' => 'Test Json Encoded Attribute',
                'backend' => \Magento\Eav\Model\Entity\Attribute\Backend\JsonEncoded::class,
                'input' => 'text',
                'required' => false,
                'global' => \Magento\Eav\Model\Entity\Attribute\ScopedAttributeInterface::SCOPE_GLOBAL,
            ]
        );
        $setup->endSetup();
    }

2. Create product with sku = 's1' and with some value for newly created attribute.
3. Save product couple times. Example console command:

   protected function execute(InputInterface $input, OutputInterface $output)
    {
        ObjectManager::getInstance()->get(State::class)->setAreaCode(Area::AREA_ADMINHTML);
        //Be sure, you have product with sku = "s1" and value for "test_json_encoded_attribute".
        $product = ObjectManager::getInstance()->get(ProductRepositoryInterface::class)->get('s1');
        $product->save();
        $product->save();
        $output->writeln($product->getCustomAttribute('test_json_encoded_attribute')->getValue());
    }
}

4. Check, no exceptions have been thrown, in console shows up correct value for "test_json_encoded_attribute".

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)