add 'danger' type to dropdown-button (#179)

Author: zeratul1

components/dropdown/dropdown-button.jsx

@@ -11,7 +11,7 @@ const ButtonGroup = Button.Group
 const DropdownButtonProps = {
   ...ButtonGroupProps,
   ...DropdownProps,
-  type: PropTypes.oneOf(['primary', 'ghost', 'dashed', 'default']).def('default'),
+  type: PropTypes.oneOf(['primary', 'ghost', 'dashed', 'danger', 'default']).def('default'),
   disabled: PropTypes.bool,
   prefixCls: PropTypes.string.def('ant-dropdown-button'),
   placement: DropdownProps.placement.def('bottomRight'),