`toggle-class` property should be passed from GlNewDropdown to BDropdown

Problem

The gl-new-dropdown wraps b-dropdown and passes attributes to it, however, it customizes the toggle-class property, so it should pass its value explicitly.

How to reproduce

Create a dropdown:

<gl-new-dropdown toggle-class="gl-w-full">

Expected

gl-w-full becomes a class of the button tag.

<div class="dropdown gl-new-dropdown ..." ...>
  <button aria-haspopup="true" aria-expanded="false" type="button" class="btn dropdown-toggle btn-default btn-md gl-button gl-w-full" ...>
    ...
  </button>
</div>

Current

gl-w-full is not used and is rendered as an attribute in the dropdown.

<div class="dropdown gl-new-dropdown ..."  toggle-class="gl-w-full">
  <button aria-haspopup="true" aria-expanded="false" type="button" class="btn dropdown-toggle btn-default btn-md gl-button" ...>
    ...
  </button>
</div>

Related to work at gitlab#214582 (closed)