Allow symbols, keywords and arbitrary cljs objects in markup

React no longer defaults to applying toString, and instead throws
confusing error messages when it encounters non-element objects.

This should make that happen less often.

src/reagent/impl/template.cljs

@@ -318,6 +318,8 @@
         (seq? x) (if (dev?)
                    (expand-seq-check x)
                    (expand-seq x))
+        (named? x) (name x)
+        (satisfies? IPrintWithWriter x) (pr-str x)
         :else x))
 
 (defn expand-seq [s]

test/reagenttest/testreagent.cljs

@@ -956,3 +956,9 @@
 (deftest test-empty-input
   (is (= "<div><input/></div>"
          (rstr [:div [:input]]))))
+
+(deftest test-object-children
+  (is (= "<p>foo bar1</p>"
+         (rstr [:p 'foo " " :bar nil 1])))
+  (is (= "<p>#&lt;Atom: 1&gt;</p>"
+         (rstr [:p (r/atom 1)]))))