Disabling sort still allows sorting using keyboard

Please make sure each of the following is true before submitting a new issue. We are a small team with limited time and it is very difficult to manage issues if they aren't reported correctly.

• This is a bug report, not a question on how to use the grid.
  Use Stack Overflow or Gitter for questions.
• You have searched the open issues to see if this bug has been filed before. We don't want duplicate issues.
• You have reviewed the tutorials and documentation to ensure you are using the grid correctly. ui-grid.info
• You are using the latest version of the grid. Older versions are not patched or supported.
• You have provided steps to recreate your bug. A plunkr is even better.

Providing EnableSorting=false in gridOptions disables sorting using mouse and removes sorting options from the column header dropdown. However, it still allows sorting using keyboard(using space bar).

Plunkr link: http://plnkr.co/edit/B6x68zlUPLKAPpmo