[Data Liberation] Wrong comment meta key values

[zaerl]

A <wp:commentmeta> fields are loaded as key/value here. The real name is meta_key/meta_value. See the WordPress core importer unit tests data here: https://github.com/WordPress/wordpress-importer/blob/master/phpunit/data/test-serialized-comment-meta.xml#L74-L75

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<b>Warning</b>:  Undefined array key "value" in <b>/wordpress/wp-content/plugins/data-liberation/src/import/WP_Entity_Importer.php</b> on line <b>1081</b><br />
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<b>Warning</b>:  Undefined array key "key" in <b>/wordpress/wp-content/plugins/data-liberation/src/import/WP_Entity_Importer.php</b> on line <b>1082</b><br />

About to fix it here: #2030. Changing the key names is enough.