Format array data without errors.

If an array ends up in a string template, we should attempt to format
the data gracefully instead of triggering an error. Imploding on ''
seemed like the safest option at hand.

This is necessary to fix issues like #7361

src/View/StringTemplate.php

@@ -233,7 +233,11 @@ class StringTemplate
         }
         $replace = [];
         foreach ($placeholders as $placeholder) {
-            $replace[] = isset($data[$placeholder]) ? $data[$placeholder] : null;
+            $replacement = isset($data[$placeholder]) ? $data[$placeholder] : null;
+            if (is_array($replacement)) {
+                $replacement = implode('', $replacement);
+            }
+            $replace[] = $replacement;
         }
         return vsprintf($template, $replace);
     }

tests/TestCase/View/StringTemplateTest.php

@@ -104,6 +104,31 @@ class StringTemplateTest extends TestCase
     }
 
     /**
+     * Formatting array data should not trigger errors.
+     *
+     * @return void
+     */
+    public function testFormatArrayData()
+    {
+        $templates = [
+            'link' => '<a href="{{url}}">{{text}}</a>'
+        ];
+        $this->template->add($templates);
+
+        $result = $this->template->format('link', [
+            'url' => '/',
+            'text' => ['example', 'text']
+        ]);
+        $this->assertEquals('<a href="/">exampletext</a>', $result);
+
+        $result = $this->template->format('link', [
+            'url' => '/',
+            'text' => ['key' => 'example', 'text']
+        ]);
+        $this->assertEquals('<a href="/">exampletext</a>', $result);
+    }
+
+    /**
      * Test loading templates files in the app.
      *
      * @return void