[Accessibility] NVDA not reading column header sorting dropdown button properly

[ashutosh100]

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.

This plunkr can be used for reference. When the keyboard focus is on the header dropdown button, it reads 'column menu menu button sub menu' which is not very intuitive.