title: How to push a content state to a current editor state? editorState.push method seems not work for me?
body: <p>there are some plain text content need to insert to exist textEditor, I try to use EditorState.push method, what I think it's work for the situation.
  I try something like this:</p>
  
  <pre><code>const { ContentState: { createFromText }, EditorState: { createWithContent, push }} = DraftJS;
  export const pushTextToCurrentEditorState = (text, editorState) =&gt; {
      const textContentState = createFromText(text);
      const newEditorState = push(editorState, textContentState, 'insert-characters');
      // debugger;
      console.log(editorStateToJSON(editorState))
      console.log(editorStateToJSON(newEditorState))
      return JSON.parse(editorStateToJSON(newEditorState));
  }
  </code></pre>
  
  <p>result is <code>newEditorState</code> is not the merge state ,but replace one, older editorState miss, <code>newEditorState</code> become brand new thing like create from the <code>text</code>.
  Is here something wrong usage? or there are some other ways to resolve the problem?</p>