Avoid leading spaces when first argument to joinClasses is falsy.

This detail is going to become more important once the idiom
`className={joinClasses(this.props.className, newClass)}` becomes more
common, as it will when we move away from `this.transferPropsTo`.

src/utils/joinClasses.js

@@ -35,7 +35,9 @@ function joinClasses(className/*, ... */) {
   if (argLength > 1) {
     for (var ii = 1; ii < argLength; ii++) {
       nextClass = arguments[ii];
-      nextClass && (className += ' ' + nextClass);
+      if (nextClass) {
+        className = (className ? className + ' ' : '') + nextClass;
+      }
     }
   }
   return className;