[fixed] added missed 'aria-label' prop type validation for 'ModalHeader'

and doc description for it

src/ModalHeader.js

@@ -13,7 +13,6 @@ class ModalHeader extends React.Component {
         { this.props.closeButton &&
           <button
             className='close'
-            aria-label={this.props['aria-label'] || 'Close'} //eslint-disable-line react/prop-types
             onClick={this.props.onHide}
           >
             <span aria-hidden="true">
@@ -31,6 +30,11 @@ class ModalHeader extends React.Component {
 ModalHeader.__isModalHeader = true;
 
 ModalHeader.propTypes = {
+  /**
+   * The 'aria-label' attribute is used to define a string that labels the current element.
+   * It is used for Assistive Technology when the label text is not visible on screen.
+   */
+  'aria-label': React.PropTypes.string,
   /**
    * A css class applied to the Component
    */
@@ -47,6 +51,7 @@ ModalHeader.propTypes = {
 };
 
 ModalHeader.defaultProps = {
+  'aria-label': 'Close',
   modalClassName: 'modal-header',
   closeButton: false
 };