Xlsx Writer Rich Text and TYPE_STRING

Fix #476. Another in the "better late than never" series, closed as stale in June 2018. Xlsx Writer expects cells containing RichText to have DataType `TYPE_INLINE`; but the spreadsheet associated with the issue has the cell defined as `TYPE_STRING`. Change Writer to handle RichText TYPE_STRING appropriately.

src/PhpSpreadsheet/Writer/Xlsx/Worksheet.php

@@ -1528,7 +1528,7 @@ private function writeCell(XMLWriter $objWriter, PhpspreadsheetWorksheet $worksh
 
                     break;
                 case 's':            // String
-                    $this->writeCellString($objWriter, $mappedType, $cellValueString, $flippedStringTable);
+                    $this->writeCellString($objWriter, $mappedType, ($cellValue instanceof RichText) ? $cellValue : $cellValueString, $flippedStringTable);
 
                     break;
                 case 'f':            // Formula

tests/PhpSpreadsheetTests/Writer/Xlsx/Issue476Test.php

@@ -0,0 +1,30 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PhpOffice\PhpSpreadsheetTests\Writer\Xlsx;
+
+use PhpOffice\PhpSpreadsheet\Reader\Xlsx as XlsxReader;
+use PhpOffice\PhpSpreadsheet\RichText\RichText;
+use PhpOffice\PhpSpreadsheetTests\Functional\AbstractFunctional;
+
+class Issue476Test extends AbstractFunctional
+{
+    public function testIssue476(): void
+    {
+        // RichText written in a way usually used only by strings.
+        $reader = new XlsxReader();
+        $spreadsheet = $reader->load('tests/data/Writer/XLSX/issue.476.xlsx');
+
+        $reloadedSpreadsheet = $this->writeAndReload($spreadsheet, 'Xlsx');
+        $spreadsheet->disconnectWorksheets();
+
+        $sheet = $reloadedSpreadsheet->getActiveSheet();
+        $richText = $sheet->getCell('A1')->getValue();
+        self::assertInstanceOf(RichText::class, $richText);
+        $plainText = $richText->getPlainText();
+        self::assertSame("Art. 1A of the Geneva Refugee Convention and Protocol or other international or national instruments.\n", $plainText);
+
+        $reloadedSpreadsheet->disconnectWorksheets();
+    }
+}