Support for missing styles XML file in XLSX

Some files don't have a "styles.xml" file. Excel supports these files, Spout should do too.

tests/Spout/Reader/XLSX/ReaderTest.php

@@ -473,6 +473,19 @@ class ReaderTest extends TestCase
         $this->assertEquals($expectedRows, $allRows);
     }
 
+    /**
+     * @return void
+     */
+    public function testReadShouldSupportMissingStylesXMLFile()
+    {
+        $allRows = $this->getAllRowsForFile('file_with_no_styles_in_workbook_xml.xlsx');
+
+        $expectedRows = [
+            ['s1--A1', 's1--B1'],
+        ];
+        $this->assertEquals($expectedRows, $allRows);
+    }
+
     /**
      * @return void
      */