Can set class attribute in array. (#521)

In case of you want to add class by condition.

```
Form::text('name', null, ['class' => [
    'form-control-,
    $error ? 'is-invalid' : '',
]]);
```

src/HtmlBuilder.php

@@ -455,6 +455,10 @@ protected function attributeElement($key, $value)
             return $value ? $key : '';
         }
 
+        if (is_array($value) && $key === 'class') {
+            return 'class="' . implode(' ', $value) . '"';
+        }
+
         if (! is_null($value)) {
             return $key . '="' . e($value, false) . '"';
         }

tests/FormBuilderTest.php

@@ -792,6 +792,18 @@ public function testBooleanAttributes()
         $this->assertEquals('<textarea readonly name="test" cols="50" rows="10"></textarea>', $input);
     }
 
+    public function testArrayClassAttributes()
+    {
+        $input = $this->formBuilder->text('test', null, ['class' => ['class-a', 'class-b']]);
+        $this->assertEquals('<input class="class-a class-b" name="test" type="text">', $input);
+
+        $input = $this->formBuilder->text('test', null, ['class' => [
+            'class-a',
+            false ? 'class-b' : 'class-c'
+        ]]);
+        $this->assertEquals('<input class="class-a class-c" name="test" type="text">', $input);
+    }
+
     protected function setModel(array $data, $object = true)
     {
         if ($object) {

tests/HtmlBuilderTest.php

@@ -151,4 +151,18 @@ public function testBooleanAttributes()
 
         $this->assertEquals('', trim($result2));
     }
+
+    public function testArrayClassAttributes()
+    {
+        $result = $this->htmlBuilder->attributes(['class' => ['class-a', 'class-b']]);
+
+        $this->assertEquals('class="class-a class-b"', trim($result));
+
+        $result = $this->htmlBuilder->attributes(['class' => [
+            'class-a',
+            false ? 'class-b' : 'class-c'
+        ]]);
+
+        $this->assertEquals('class="class-a class-c"', trim($result));
+    }
 }