[fixed] Allow empty classNames for body (#720)

* [fixed] Allow empty classNames for body

* Regenerate package-lock.json to fix event stream issue

Author: jitinmaher

docs/README.md

@@ -66,6 +66,7 @@ import ReactModal from 'react-modal';
   className="ReactModal__Content"
   /*
      String className to be applied to the document.body (must be a constant string).
+     This attribute when set as `null` doesn't add any class to document.body.
      See the `Styles` section for more details.
   */
   bodyOpenClassName="ReactModal__Body--open"

docs/styles/classes.md

@@ -48,6 +48,8 @@ require a complex system to manage which class name should be added to or
 removed from `document.body` from which modal (if using multiple modals
 simultaneously).  The default value is `ReactModal__Body--open`.
 
+`bodyOpenClassName` when set as `null` doesn't add any class to `document.body`.
+
 `bodyOpenClassName` can support adding multiple classes to `document.body` when
 the modal is open. Add as many class names as you desire, delineated by spaces.