Spreadhseet encodings monkey-patch, fixes "can't modify a frozen string" error.

spreadsheet_encodings.rb

# monkey patch frozen string bug, see https://github.com/dblock/spreadsheet/commit/164dcfbb24097728f1a7453702c270107e725b7c
module Spreadsheet
  module Encodings
    def client string, internal='UTF-16LE'
      string = string.dup
      string.force_encoding internal
      string.encode Spreadsheet.client_encoding
    end
    def internal string, client=Spreadsheet.client_encoding
      string = string.dup
      string.force_encoding client
      string.encode('UTF-16LE').force_encoding('ASCII-8BIT')
    end
    def utf8 string, client=Spreadsheet.client_encoding
      string = string.dup
      string.force_encoding client
      string.encode('UTF-8')
    end
  end
end