Add "data-key" to root div for the whole document (#759)

* Add "data-key" to div for the whole document

* Fix unit tests for rendering

src/components/content.js

@@ -836,6 +836,7 @@ class Content extends React.Component {
         data-slate-editor
         key={this.tmp.forces}
         ref={this.ref}
+        data-key={document.key}
         contentEditable={!readOnly}
         suppressContentEditableWarning
         className={className}

test/rendering/index.js

@@ -59,6 +59,7 @@ function clean(html) {
   $.root().children().removeAttr('autocorrect')
   $.root().children().removeAttr('spellcheck')
   $.root().children().removeAttr('style')
+  $.root().children().removeAttr('data-gramm')
 
   return $.html()
 }