code: function stringifyClass(classAttr) {
    if (typeof classAttr === 'string') {
      return classAttr;
    }
    var classes = [];
    for (var key in classAttr) {
      if (classAttr[key]) {
        classes.push(key);
      }
    }
    return classes.join(' ');
  }
docs: Returns a `'class1 class3' ...'` string from
  a `{ class1: true, class2: false, class3: true }` object.
  
  @param  Object className The keys/values object to stringify.
  @return String           The corresponding string.