This bugfix resolves an issue in `export_wp()` with featured images.
When using Tools > Export and selecting either Posts or Pages (with or without a specific author), the resulting XML file now includes a XML item for each post|page's featured image attachment and its metadata.
Uses same chunking (for performance) and code patterns from existing code in the same file.
Adds a new test class for `export_wp()` with code coverage specific to this bugfix.
Follow-up to [34326], [14444], [6375], [6335].
Props billseymour, nateallen, petitphp, hellofromTonya, duck_, jane, rcain, jghazally, jghazally, smub, batmoo, axwax, creativeslice, dlocc, nacin, wonderboymusic, ganon, SergeyBiryukov, hlashbrooke, chriscct7, fischfood, hifidesign, ankit-k-gupta, 5um17, shailu25, huzaifaalmesbah, mukesh27.
Fixes#17379.
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git-svn-id: http://core.svn.wordpress.org/trunk@57182 1a063a9b-81f0-0310-95a4-ce76da25c4cd
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