Better support utf-8-with-signature and utf-8-hfs in HTML

* lisp/international/mule.el (sgml-html-meta-auto-coding-function):
Support UTF-8 with BOM and utf-8-hfs as variants of UTF-8, and
obey the buffer's encoding if it is one of these variants, instead
of re-encoding in UTF-8 proper.  (Bug#20623)

lisp/international/mule.el

@@ -2544,7 +2544,17 @@ This function is intended to be added to `auto-coding-functions'."
       (let* ((match (match-string 2))
 	     (sym (intern (downcase match))))
 	(if (coding-system-p sym)
-	    sym
+            ;; If the encoding tag is UTF-8 and the buffer's
+            ;; encoding is one of the variants of UTF-8, use the
+            ;; buffer's encoding.  This allows, e.g., saving an
+            ;; HTML file as UTF-8 with BOM when the tag says UTF-8.
+            (let ((sym-type (coding-system-type sym))
+                  (bfcs-type
+                   (coding-system-type buffer-file-coding-system)))
+              (if (and (coding-system-equal 'utf-8 sym-type)
+                       (coding-system-equal 'utf-8 bfcs-type))
+                  buffer-file-coding-system
+		sym))
 	  (message "Warning: unknown coding system \"%s\"" match)
 	  nil)))))