Dropdown: don't ignore keydown from input or textarea

Created by: gpakosz

I'm reopening an issue for v4, as #15757 (closed) got closed without resolution for v3.

At the time I opened the issue, v4 was still filtering key down events for input or textarea:

static _dataApiKeydownHandler(event) {
  if (!/(38|40|27|32)/.test(event.which) ||
     /input|textarea/i.test(event.target.tagName)) {
    return
  }

In my opinion, a common use of an <input type="text"> and dropdown combination would be displaying search results as a dropdown ala http://developer.github.com:

In that respect, can you please make the behavior configurable (if you don't want to revert the default behavior).

Thank you.