Add missing image namespace XML header

pysitemap/format_processors/xml.py

@@ -15,7 +15,8 @@ class XMLWriter():
             await writer(
                 '<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"'
                 ' xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"'
-                ' xsi:schemaLocation="http://www.sitemaps.org/schemas/sitemap/0.9 http://www.sitemaps.org/schemas/sitemap/0.9/sitemap.xsd">\n')
+                ' xsi:schemaLocation="http://www.sitemaps.org/schemas/sitemap/0.9 http://www.sitemaps.org/schemas/sitemap/0.9/sitemap.xsd"'
+                ' xmlns:image="http://www.google.com/schemas/sitemap-image/1.1">\n')
             await aiodf.fsync()
             for data in urls: