Sort stop working when combining map and fixed items

[JonathanWi]

Hey there,

As referenced in this issue, displaying sortable items using map and adding a fixed items result in the component breaking.

As stated, this can be easily replicated in the demo by replacing the map render with :

<div className="dynamic-demo">
        <button onClick={::this.handleAddElement}>Add 1 element</button>
        <Sortable key={this._sortableKey} onSort={::this.handleSort}>
          {this.state.arr.map(renderItem, this)}
          <span>Hello</span>
        </Sortable>
      </div>

Any help here would be really appreciated!