Fixes #7 Fix Placeholder view helper return value

src/Helper/Placeholder.php

@@ -42,9 +42,7 @@ class Placeholder extends AbstractHelper
     public function __invoke($name = null)
     {
         if ($name === null) {
-            throw new InvalidArgumentException(
-                'Placeholder: missing argument. $name is required by placeholder($name)'
-            );
+            return $this;
         }
 
         $name = (string) $name;

test/Helper/PlaceholderTest.php

@@ -58,6 +58,17 @@ public function testSetView()
         $this->assertSame($view, $this->placeholder->getView());
     }
 
+    /**
+     * @return void
+     */
+    public function testContainerExists()
+    {
+        $this->placeholder->__invoke('foo');
+        $containerExists = $this->placeholder->__invoke()->containerExists('foo');
+
+        $this->assertTrue($containerExists);
+    }
+
     /**
      * @return void
      */
@@ -67,6 +78,15 @@ public function testPlaceholderRetrievesContainer()
         $this->assertInstanceOf(AbstractContainer::class, $container);
     }
 
+    /**
+     * @return void
+     */
+    public function testPlaceholderRetrievesItself()
+    {
+        $container = $this->placeholder->__invoke();
+        $this->assertSame($container, $this->placeholder);
+    }
+
     /**
      * @return void
      */
@@ -101,4 +121,15 @@ public function testClearContainersRemovesAllContainers()
         $this->assertFalse($this->placeholder->containerExists('foo'));
         $this->assertFalse($this->placeholder->containerExists('bar'));
     }
+
+    /**
+     * @return void
+     */
+    public function testGetContainerRetrievesTheCorrectContainer()
+    {
+        $container1 = $this->placeholder->__invoke('foo');
+        $container2 = $this->placeholder->__invoke()->getContainer('foo');
+
+        $this->assertSame($container1, $container2);
+    }
 }