Remove unnecessary uniqueness check in get_attachment_taxonomies().

Running the taxonomy array through `array_unique()` is unnecessary
when the function returns objects, because the associative keys already
ensure uniqueness.

This also fixes a bug when running `get_attachment_taxonomies()` in
HHVM, which doesn't like casting objects to strings for the purposes
of `array_unique()`.

Props swissspidy.
See #37368.

git-svn-id: https://develop.svn.wordpress.org/trunk@38437 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/media.php

@@ -2749,7 +2749,11 @@ function get_attachment_taxonomies( $attachment, $output = 'names' ) {
 		}
 	}
 
-	return array_unique($taxonomies);
+	if ( 'names' === $output ) {
+		$taxonomies = array_unique( $taxonomies );
+	}
+
+	return $taxonomies;
 }
 
 /**