Fix Formatters::valueToString when is an array

Signed-off-by: davidarendsen <davidarendsen@hey.com>

tests/FormattersTest.php

@@ -0,0 +1,21 @@
+<?php
+declare(strict_types=1);
+
+use Arendsen\FluxQueryBuilder\Formatters;
+use PHPUnit\Framework\TestCase;
+
+final class FormattersTest extends TestCase {
+
+    public function testAssociativeArrayNested()
+    {
+        $array = [
+            'columns' => ['foo', 'bar'],
+            'mode' => 'by'
+        ];
+
+        $expected = 'columns: ["foo", "bar"], mode: "by"';
+
+        $this->assertEquals($expected, Formatters::toFluxArrayString($array));
+    }
+
+}