Fix/image import non ascii

[preda-bogdan]

Summary

Similar to the spaces fix the URL was not passing the filter function. Improved the URL sanitization before running the filter so that URLs are converted to ASCII and escaped. Same as it happens inside the browser. This should also allow for all valid URLs to pass the check.

Added test for this specific case with internalized URLs.

Will affect the visual aspect of the product

NO

Screenshots

Test instructions

1. Try to import a feed with internalized image URLs
2. Check that the images are being imported.
3. All tests should pass.

Check before Pull Request is ready:

• [x] I have written a test and included it in this PR
• [x] I have run all tests and they pass
• [x] The code passes when running the PHP CodeSniffer
• [x] Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• [x] Security and Sanitization requirements have been followed
• [x] I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)

Closes #928.

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:tada: This PR is included in version 4.4.9 :tada:

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