Fixed warning header as array in YAML tests generator

Author: ezimuel

util/template/test/warnings

@@ -3,5 +3,5 @@
         $this->assertTrue(isset($headers['Warning']));
         $expectedHeaders = :expected;
         foreach ($expectedHeaders as $expected) {
-            $this->assertStringContainsString($expected, $headers['Warning'][0]);
+            $this->assertStringContainsString($expected, implode('', $headers['Warning']));
         }