More tests

Author: Imangazaliev

src/DiDom/Document.php

@@ -489,7 +489,11 @@ public function getElement()
      */
     public function toElement()
     {
-        return new Element($this->getElement());
+        if ($this->document->documentElement === null) {
+            throw new RuntimeException('Cannot convert empty document to Element');
+        }
+
+        return new Element($this->document->documentElement);
     }
 
     /**

tests/DiDom/DocumentTest.php

@@ -477,6 +477,16 @@ public function testGetElement()
         $this->assertInstanceOf('DOMElement', $document->getElement());
     }
 
+    /**
+     * @expectedException RuntimeException
+     */
+    public function testEmptyDocumentToElement()
+    {
+        $document = new Document();
+
+        $document->toElement();
+    }
+
     public function testToElement()
     {
         $html = $this->loadFixture('posts.html');