fix(b-dropdown): Sets correct `aria-haspopup` attribute for the toggle button

Updates aria-haspopup attribute on the toggle button to be set based on the role property provided for the dropdown popup.

Based on the specs: https://www.w3.org/TR/wai-aria-1.1/#aria-haspopup https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Attributes/aria-haspopup

Authors MUST ensure that the role of the element that serves as the container for the popup content is menu, listbox, tree, grid, or dialog, and that the value of aria-haspopup matches the role of the popup container.