v4 custom-select does not support "multiple"

[ghost]

The current bootstrap v4 beta 6 comes with a new custom-select class, but it appears broken when the "multiple" attribute is also defined.

For example:
<select multiple class="form-control" id="exampleSelect2">

changes into
<select multiple class="custom-select" id="exampleSelect2">

Maybe the documentation could mention that multiple is not supported, or if it is not too much work, to implement custom-select a bit better for displaying on a multiple select.

Thank you.

[C-Lodder]

PR: #23055