fix(iframe-block): add a trailing slash to iframe URL (#906)

without the trailing slash, the iframe URL do a redirection to add the slash and set the request on
http which raise a mixed content issue, adding the trailing slash would stop the redirection and
minimize the calls to the server

src/blocks/iframe/class-wp-rest-newspack-iframe-controller.php

@@ -165,7 +165,7 @@ private function process_iframe_archive( $request, $iframe_folder, $media_path )
 			// check if iframe entry file is there.
 			if ( file_exists( path_join( $iframe_path, self::IFRAME_ENTRY_FILE ) ) ) {
 				$response = [
-					'src' => $wp_upload_dir['url'] . self::IFRAME_UPLOAD_DIR . $iframe_folder,
+					'src' => $wp_upload_dir['url'] . self::IFRAME_UPLOAD_DIR . $iframe_folder . DIRECTORY_SEPARATOR,
 					'dir' => path_join( $wp_upload_dir['subdir'] . self::IFRAME_UPLOAD_DIR, $iframe_folder ),
 				];
 
@@ -186,7 +186,7 @@ function( $file ) {
 
 				if ( 1 === count( $archive_folders ) && file_exists( path_join( $archive_folders[0], self::IFRAME_ENTRY_FILE ) ) ) {
 					$response = [
-						'src' => $wp_upload_dir['url'] . self::IFRAME_UPLOAD_DIR . $iframe_folder . DIRECTORY_SEPARATOR . basename( $archive_folders[0] ),
+						'src' => $wp_upload_dir['url'] . self::IFRAME_UPLOAD_DIR . $iframe_folder . DIRECTORY_SEPARATOR . basename( $archive_folders[0] ) . DIRECTORY_SEPARATOR,
 						'dir' => path_join( $wp_upload_dir['subdir'] . self::IFRAME_UPLOAD_DIR, $iframe_folder . DIRECTORY_SEPARATOR ),
 					];