From js/ReactDOMServer -> react-dom/server's renderToString

Use proper require of react-dom/server and invoke its renderToString,
instead of accessing the global js/ReactDOMServer, which might not be available.

I had troubles making sure that `js/ReactDOMServer` and thus couldn't render to a string.
With this change, it works fine.

src/main/com/fulcrologic/fulcro/dom.cljs

@@ -8,6 +8,7 @@
     [com.fulcrologic.fulcro.components :as comp]
     ["react" :as react]
     ["react-dom" :as react.dom]
+    ["react-dom/server" :as react-dom-server]
     [goog.object :as gobj]
     [goog.dom :as gdom]
     [com.fulcrologic.fulcro.dom.inputs :as inputs]
@@ -72,9 +73,9 @@
   (react.dom/render component el))
 
 (defn render-to-str
-  "Equivalent to React.renderToString. NOTE: You must make sure js/ReactDOMServer is defined (e.g. require cljsjs.react.dom.server) to use this function."
+  "Equivalent to React.renderToString."
   [c]
-  (js/ReactDOMServer.renderToString c))
+  (react-dom-server/renderToString c))
 
 (defn node
   "Returns the dom node associated with a component's React ref."