email-gravatar: fix html syntax issues

an attribute value specification must be an attribute value literal
unless SHORTTAG YES is specified

filters/email-gravatar.lua

@@ -15,7 +15,7 @@ function filter_open(email, page)
 end
 
 function filter_close()
-	html("<img src='//www.gravatar.com/avatar/" .. md5 .. "?s=13&amp;d=retro' width=13 height=13 alt='Gravatar' /> " .. buffer)
+	html("<img src='//www.gravatar.com/avatar/" .. md5 .. "?s=13&amp;d=retro' width='13' height='13' alt='Gravatar' /> " .. buffer)
 	return 0
 end
 

filters/email-gravatar.py

@@ -36,4 +36,4 @@ sys.stdout = codecs.getwriter("utf-8")(sys.stdout.detach())
 md5 = hashlib.md5(email.encode()).hexdigest()
 text = sys.stdin.read().strip()
 
-print("<img src='//www.gravatar.com/avatar/" + md5 + "?s=13&amp;d=retro' width=13 height=13 alt='Gravatar' /> " + text)
+print("<img src='//www.gravatar.com/avatar/" + md5 + "?s=13&amp;d=retro' width='13' height='13' alt='Gravatar' /> " + text)