Do not downcase string tags in HTML/XML artifacts

If the tag of an S-XML expression is given as a string that is a sign
that the author actually does want to preserve the case.  Downcasing
should only be performed for symbol tags.  An example of wanting to
preserve the case is the "pubDate" tag in RSS feeds.

src/hssg/artifacts/html.lisp

@@ -45,7 +45,7 @@
   (declare (type atom symbol)
            (optimize (speed 3) (safety 0)))
   (etypecase symbol
-    (string (string-downcase symbol))
+    (string symbol)
     (symbol (string-downcase (symbol-name symbol)))
     (atom   (string-downcase (princ-to-string symbol)))))