Merge pull request #620 from HashNotAdam/explicitly_define_mutable_strings

Explicitly define mutable strings

Author: simonoff

lib/roo/formatters/yaml.rb

@@ -13,7 +13,7 @@ def to_yaml(prefix = {}, from_row = nil, from_column = nil, to_row = nil, to_col
         from_column ||= first_column(sheet)
         to_column ||= last_column(sheet)
 
-        result = "--- \n"
+        result = String.new("--- \n")
         from_row.upto(to_row) do |row|
           from_column.upto(to_column) do |col|
             next if empty?(row, col, sheet)

test/roo/test_open_office.rb

@@ -224,7 +224,7 @@ def test_bug_ric
       assert workbook.empty?("C", 1)
       assert workbook.empty?("D", 1)
       expected = 1
-      letter = "e"
+      letter = String.new("e")
       while letter <= "u"
         assert_equal expected, workbook.cell(letter, 1)
         letter.succ!