add ability to set codepage explicitly for BIFF5

If BIFF5 excel 95 file doesn't have codepage record, the default codepage CP1252 is used and can't be change.
That causes to problems with decoding cyrillic text.

Author: boltunoreh

CHANGELOG.md

@@ -5,6 +5,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com)
 and this project adheres to [Semantic Versioning](https://semver.org).
 
+## [Unreleased]
+
+### Added
+
+- Add ability to set codepage explicitly for BIFF5 [#1018](https://github.com/PHPOffice/PhpSpreadsheet/issues/1018)
+
 ## [1.13.0] - 2020-05-31
 
 ### Added

src/PhpSpreadsheet/Reader/Xls.php

@@ -439,6 +439,20 @@ public function canRead($pFilename)
         }
     }
 
+    /**
+     * @param string $codepage
+     *
+     * @throws PhpSpreadsheetException
+     */
+    public function setCodepage(string $codepage): void
+    {
+        if (!CodePage::validate($codepage)) {
+            throw new PhpSpreadsheetException('Unknown codepage: ' . $codepage);
+        }
+
+        $this->codepage = $codepage;
+    }
+
     /**
      * Reads names of the worksheets from a file, without parsing the whole file to a PhpSpreadsheet object.
      *
@@ -640,7 +654,7 @@ public function load($pFilename)
 
         // initialize
         $this->pos = 0;
-        $this->codepage = 'CP1252';
+        $this->codepage = $this->codepage ?: CodePage::DEFAULT_CODE_PAGE;
         $this->formats = [];
         $this->objFonts = [];
         $this->palette = [];

src/PhpSpreadsheet/Shared/CodePage.php

@@ -6,6 +6,8 @@
 
 class CodePage
 {
+    public const DEFAULT_CODE_PAGE = 'CP1252';
+
     private static $pageArray = [
         0 => 'CP1252', //    CodePage is not always correctly set when the xls file was saved by Apple's Numbers program
         367 => 'ASCII', //    ASCII
@@ -65,13 +67,25 @@ class CodePage
         65001 => 'UTF-8', //    Unicode (UTF-8)
     ];
 
+    /**
+     * @param string $codePage
+     *
+     * @return bool
+     */
+    public static function validate(string $codePage): bool
+    {
+        return (in_array($codePage, self::$pageArray, true));
+    }
+
     /**
      * Convert Microsoft Code Page Identifier to Code Page Name which iconv
      * and mbstring understands.
      *
      * @param int $codePage Microsoft Code Page Indentifier
      *
      * @return string Code Page Name
+     *
+     * @throws PhpSpreadsheetException
      */
     public static function numberToName(int $codePage): string
     {