Make the attribute id take priority over the tag.

This matches the behavior of Reagent and the original Hiccup.

For example, in `[:div#foo {:id "bar"}]`, the id would be bar and not
foo.

src/lambdaisland/hiccup.clj

@@ -43,7 +43,8 @@
                              (into {} (filter val m))
                              {})
                     :content (enlive/flatmap #(nodify % opts) (if (attr-map? m) ms more))}
-              node (if id (assoc-in node [:attrs :id] id) node)
+              node (if (and id (not (contains? (:attrs node) :id))) 
+                     (assoc-in node [:attrs :id] id) node)
               node (if (seq classes)
                      (update-in node
                                 [:attrs :class]