[Backport] Special price date from issue resolve

[tufahu]

Original Pull Request

#18578
Fixed issue of 2.2.6 "Special price date from" Failed to parse time string

Description (*)

In magento 2.2.6 when Account locale set to it_IT and admin user create one new product and set it's special price then it will gives error while saving the product that failed to parse time string. In this Pull Request I have fixed this issue. By modifing date format before it's parse to Date object.

Fixed Issues (if relevant)

1. 2.2.6 "Special price date from" Failed to parse time string #18158: 2.2.6 "Special price date from" Failed to parse time string

Steps to reproduce

1. Magento 2.2.6 create a new product with a price
2. Edit the new product and add a special price

Expected result

1. The product has been saved

Actual result

1. DateTime::__construct(): Failed to parse time string (20/09/2018) at position 0 (2): Unexpected character

Manual testing scenarios (*)

1. Account locale set to it_IT if it's not set.
2. Add one new product and save. Than edit the product by adding special price and set from date as well. Press Save.
3. Product Save successfully that message will appear.
4. Now, Set another account locale and repeat the proceess, product will save successfully again.

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 @tufahu. Thank you for your contribution
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[ghost]

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

Hi @VladimirZaets, thank you for the review.
ENGCOM-4290 has been created to process this Pull Request

[sivaschenko]

@magento-engcom-team give me test instance

[magento-engcom-team]

Hi @sivaschenko. Thank you for your request. I'm working on Magento instance for you

[magento-engcom-team]

Hi @sivaschenko, here is your new Magento instance.
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[ghost]

Hi @tufahu, thank you for your contribution!
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