Twig implementation: passing array to set tag creates a TwigReference object [#1916834]

Problem/Motivation

Twig's set tag can be used to set variables within the context of a template. This can be useful in many cases, for example passing arguments to t().

Without set tag:

<p class="submitted">{{ 'by !author on !date' | t({'!author': author, '!date': created}) }}</p>

With set tag:

{% set args = {'!author': author, '!date': created} %}
<p class="submitted">{{ 'by !author on !date' | t(args) }}</p>

The following snippet will demonstrate the bug if pasted into any Twig template:

{% set args = {'!author': 'foo', '!date': 'bar'} %}
{{ 'by !author on !date' | t(args) }}

That will result in an error like this:
Warning: strtr(): The second argument is not an array in format_string() (line 1594 of core/includes/bootstrap.inc).

You can also debug right in the template and see that any array is converted into a TwigReference object. The first example is an indexed array, the second is an associative array. Strings come back fine.

{% set array = ['baz', 'bar'] %}
{% set hash = {'foo': 'bar'} %}
{% set string = "string" %}
<pre>{{ dump(array) }}</pre>
<pre>{{ dump(hash) }}</pre>
<pre>{{ dump(string) }}</pre>

object(Drupal\Core\Template\TwigReference)#626 (2) {
  ["writableRef":protected]=>
  &array(2) {
    [0]=>
    string(3) "baz"
    [1]=>
    string(3) "bar"
  }
  ["storage":"ArrayObject":private]=>
  array(2) {
    [0]=>
    string(3) "baz"
    [1]=>
    string(3) "bar"
  }
}
object(Drupal\Core\Template\TwigReference)#604 (2) {
  ["writableRef":protected]=>
  &array(1) {
    ["foo"]=>
    string(3) "bar"
  }
  ["storage":"ArrayObject":private]=>
  array(1) {
    ["foo"]=>
    string(3) "bar"
  }
}
string(6) "string"