Fix #401 if jsprops is nil before adding key, key is lost

If jsprops was nil before key oset call, oset created a new object for
the jsprops, but because the return value from oset was not used, the
object was lost.

src/reagent/impl/template.cljs

@@ -323,9 +323,10 @@
   (let [props (nth argv 1 nil)
         hasprops (or (nil? props) (map? props))
         jsprops (convert-prop-value (if hasprops props))
+        jsprops (if-some [key (key-from-vec argv)]
+                  (oset jsprops "key" key)
+                  jsprops)
         first-child (+ 1 (if hasprops 1 0))]
-    (when-some [key (key-from-vec argv)]
-      (oset jsprops "key" key))
     (make-element argv react/Fragment jsprops first-child)))
 
 (defn adapt-react-class

test/reagenttest/testreagent.cljs

@@ -1189,8 +1189,12 @@
                      [:div "hello"]
                      [:div "world"]]
                     ^{:key 2}
-                    [children])])]
-      (is (= "<div><div>hello</div><div>world</div><div>foo</div></div>"
+                    [children]
+                    ^{:key 3}
+                    [:<>
+                     [:div "1"]
+                     [:div "2"]])])]
+      (is (= "<div><div>hello</div><div>world</div><div>foo</div><div>1</div><div>2</div></div>"
              (as-string [comp]))))))
 
 (defonce my-context (react/createContext "default"))