Handle General number format as non date (#221)

If the number format is set to General (id = 0), do no try to format the value as a date

tests/Spout/Reader/XLSX/Helper/StyleHelperTest.php

@@ -59,6 +59,16 @@ class StyleHelperTest extends \PHPUnit_Framework_TestCase
         $this->assertFalse($shouldFormatAsDate);
     }
 
+    /**
+     * @return void
+     */
+    public function testShouldFormatNumericValueAsDateWithGeneralFormat()
+    {
+        $styleHelper = $this->getStyleHelperMock([[], ['applyNumberFormat' => true, 'numFmtId' => 0]]);
+        $shouldFormatAsDate = $styleHelper->shouldFormatNumericValueAsDate(1);
+        $this->assertFalse($shouldFormatAsDate);
+    }
+
     /**
      * @return void
      */