Fix react warning in react-components

Summary:
This fixes a recent react warning that would show up every time you
opened a Perseus editor locally. 😱

I'm not sure whether we're allowed to use object rest spread syntax in this
repo, so I'm just using normal ES6 stuff.

Test Plan:
 - Visit http://localhost:9000/?article#content=%5B%7B%22content%22%3A%22Here%27s%20the%20type%20of%20problem%20you%20might%20see%20on%20the%20AP%20Statistics%20exam%20where%20you%20have%20to%20use%20the%20sampling%20distribution%20of%20a%20sample%20proportion.%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%5D
 - See no react warning!

Reviewers: john, joshuan

Reviewed By: john

Subscribers: tom

Differential Revision: https://phabricator.khanacademy.org/D40017

Author: xymostech

js/drag-target.jsx

@@ -64,9 +64,13 @@ const DragTarget = React.createClass({
         const opacity = this.state.dragHover ? {"opacity": 0.3} : {};
         const Component = this.props.component;
 
+        const forwardProps = Object.assign({}, this.props);
+        delete forwardProps.component;
+        delete forwardProps.shouldDragHighlight;
+
         return (
             <Component
-                {...this.props}
+                {...forwardProps}
                 style={Object.assign({}, this.props.style, opacity)}
                 onDrop={this.handleDrop}
                 onDragEnd={this.handleDragEnd}