Add and improve test coverage

tests/Spout/Writer/WriterFactoryTest.php

@@ -0,0 +1,21 @@
+<?php
+
+namespace Box\Spout\Writer;
+
+/**
+ * Class WriterFactoryTest
+ *
+ * @package Box\Spout\Writer
+ */
+class WriterFactoryTest extends \PHPUnit_Framework_TestCase
+{
+    /**
+     * @expectedException \Box\Spout\Common\Exception\UnsupportedTypeException
+     *
+     * @return void
+     */
+    public function testCreateWriterShouldThrowWithUnsupportedType()
+    {
+        WriterFactory::create('unsupportedType');
+    }
+}