Fix/open office gsub replacement

CHANGELOG.md

@@ -1,4 +1,5 @@
 ##  Unreleased
+- Fix gsub! usage for open office documents on a frozen string [580](https://github.com/roo-rb/roo/pull/580)
 
 ### Changed/Added
 - Roo::Base#each_with_pagename returns Enumerator Object [576](https://github.com/roo-rb/roo/pull/576)

lib/roo/open_office.rb

@@ -517,7 +517,7 @@ def read_cells(sheet = default_sheet)
                         str_v += child.content #.text
                       end
                     end
-                    str_v.gsub!(/'/, "'") # special case not supported by unescapeHTML
+                    str_v = str_v.gsub(/'/, "'") # special case not supported by unescapeHTML
                     str_v = CGI.unescapeHTML(str_v)
                   end # == 'p'
                 end

test/test_roo.rb

@@ -282,6 +282,18 @@ def test_cell_multiline
     end
   end
 
+  def test_apostrophe_replacement
+    with_each_spreadsheet(:name=>'apostrophe', :format=>[:openoffice]) do |oo|
+      assert_equal "'", oo.cell(1,1)
+    end
+  end
+
+  def test_frozen_string_usage
+    with_each_spreadsheet(:name=>'frozen_string', :format=>[:openoffice]) do |oo|
+      assert_equal "", oo.cell(1,1)
+    end
+  end
+
   def test_row_whitespace
     # auf dieses Dokument habe ich keinen Zugriff TODO:
     # TODO: No access to document whitespace?