feat: add support for empty classNames (#481)

Hey!

I think it would be quite usefull if `classNames` prop would be optional, and it would still work without it.

In our use-case we are using it together with styled components and it's quite annoying to add those prefixes all the time, when component styles are already scoped.

Also if you pass empty string, it adds classes beginning with `-`.

src/CSSTransition.js

@@ -71,6 +71,9 @@ const removeClass = (node, classes) => node && classes && classes.split(' ').for
  * `*-active` classes represent which styles you want to animate **to**.
  */
 class CSSTransition extends React.Component {
+  static defaultProps = {
+    classNames: ''
+  }
   onEnter = (node, appearing) => {
     const { className } = this.getClassNames(appearing ? 'appear' : 'enter')
 
@@ -140,15 +143,17 @@ class CSSTransition extends React.Component {
 
   getClassNames = (type) => {
     const { classNames } = this.props;
+    const isStringClassNames = typeof classNames === 'string';
+    const prefix = isStringClassNames && classNames ? classNames + '-' : '';
 
-    let className = typeof classNames !== 'string' ?
-      classNames[type] : classNames + '-' + type;
+    let className = isStringClassNames ?
+      prefix + type : classNames[type]
 
-    let activeClassName = typeof classNames !== 'string' ?
-      classNames[type + 'Active'] : className + '-active';
+    let activeClassName = isStringClassNames ?
+      className + '-active' : classNames[type + 'Active'];
 
-    let doneClassName = typeof classNames !== 'string' ?
-      classNames[type + 'Done'] : className + '-done';
+    let doneClassName = isStringClassNames ?
+      className + '-done' : classNames[type + 'Done'];
 
     return {
       className,

test/CSSTransition-test.js

@@ -279,5 +279,38 @@ describe('CSSTransition', () => {
         }
       });
     });
+
+    it('should support empty prefix', done => {
+      let count = 0;
+
+      const instance = mount(
+        <CSSTransition
+          in
+          timeout={10}
+        >
+          <div/>
+        </CSSTransition>
+      )
+
+      instance.setProps({
+        in: false,
+
+        onExit(node){
+          count++;
+          expect(node.className).toEqual('exit');
+        },
+
+        onExiting(node){
+          count++;
+          expect(node.className).toEqual('exit exit-active');
+        },
+
+        onExited(node){
+          expect(node.className).toEqual('exit-done');
+          expect(count).toEqual(2);
+          done();
+        }
+      });
+    });
   });
 });