[fixed] overlay classNames are maintained by overlayTrigget

fixes react-bootstrap#1049

src/Overlay.js

@@ -78,10 +78,9 @@ class Overlay extends React.Component {
         </Transition>
       );
     } else {
-      child = cloneElement(
-        child,
-        {className: classNames('in', child.className)}
-      );
+      child = cloneElement(child, {
+        className: classNames('in', child.props.className)
+      });
     }
 
     // This goes after everything else because it adds a wrapping div.

src/Position.js

@@ -1,4 +1,5 @@
 import React, { cloneElement } from 'react';
+import classNames from 'classnames';
 import domUtils from './utils/domUtils';
 import { calcOverlayPosition } from './utils/overlayPositionUtils';
 import CustomPropTypes from './utils/CustomPropTypes';
@@ -40,7 +41,7 @@ class Position extends React.Component {
   }
 
   render() {
-    const {children, ...props} = this.props;
+    const {children, className, ...props} = this.props;
     const {positionLeft, positionTop, ...arrowPosition} = this.state;
 
     const child = React.Children.only(children);
@@ -51,6 +52,7 @@ class Position extends React.Component {
         ...arrowPosition,
         positionTop,
         positionLeft,
+        className: classNames(className, child.props.className),
         style: {
           ...child.props.style,
           left: positionLeft,

test/OverlayTriggerSpec.js

@@ -39,6 +39,19 @@ describe('OverlayTrigger', function() {
     instance.state.isOverlayShown.should.be.true;
   });
 
+  it('Should maintain overlay classname', function() {
+    const instance = ReactTestUtils.renderIntoDocument(
+      <OverlayTrigger trigger='click' overlay={<div className='test-overlay'>test</div>}>
+        <button>button</button>
+      </OverlayTrigger>
+    );
+
+    const overlayTrigger = React.findDOMNode(instance);
+    ReactTestUtils.Simulate.click(overlayTrigger);
+
+    expect(document.getElementsByClassName('test-overlay').length).to.equal(1)
+  });
+
   it('Should pass transition callbacks to Transition', function (done) {
     let count = 0;
     let increment = ()=> count++;