Refactory Writer Styles to match new code organization (#433)

Decomposed old StyleHelper into StyleManager, StyleRegistry and StyleMerger.

tests/Spout/Writer/Common/Creator/StyleBuilderTest.php

@@ -0,0 +1,45 @@
+<?php
+
+namespace Box\Spout\Writer\Common\Creator\Style;
+
+use Box\Spout\Writer\Common\Entity\Style\Border;
+use Box\Spout\Writer\Common\Entity\Style\Color;
+use Box\Spout\Writer\Common\Manager\Style\StyleMerger;
+
+/**
+ * Class StyleManagerTest
+ *
+ * @package Box\Spout\Writer\Common\Creator\Style
+ */
+class StyleBuilderTest extends \PHPUnit_Framework_TestCase
+{
+    /**
+     * @return void
+     */
+    public function testStyleBuilderShouldApplyBorders()
+    {
+        $border = (new BorderBuilder())
+            ->setBorderBottom()
+            ->build();
+        $style = (new StyleBuilder())->setBorder($border)->build();
+        $this->assertTrue($style->shouldApplyBorder());
+    }
+
+    /**
+     * @return void
+     */
+    public function testStyleBuilderShouldMergeBorders()
+    {
+        $border = (new BorderBuilder())->setBorderBottom(Color::RED, Border::WIDTH_THIN, Border::STYLE_DASHED)->build();
+
+        $baseStyle = (new StyleBuilder())->setBorder($border)->build();
+        $currentStyle = (new StyleBuilder())->build();
+
+        $styleMerger = new StyleMerger();
+        $mergedStyle = $styleMerger->merge($currentStyle, $baseStyle);
+
+        $this->assertEquals(null, $currentStyle->getBorder(), 'Current style has no border');
+        $this->assertInstanceOf('Box\Spout\Writer\Common\Entity\Style\Border', $baseStyle->getBorder(), 'Base style has a border');
+        $this->assertInstanceOf('Box\Spout\Writer\Common\Entity\Style\Border', $mergedStyle->getBorder(), 'Merged style has a border');
+    }
+}