def to_xml_string(str='')
      Axlsx::sanitize(@shared_xml_string)
      str << ('<?xml version="1.0" encoding="UTF-8"?><sst xmlns="' << XML_NS << '"')
      str << (' count="' << @count.to_s << '" uniqueCount="' << unique_count.to_s << '"')
      str << (' xml:space="' << xml_space.to_s << '">' << @shared_xml_string << '</sst>')
    end
def to_xml_string(str='')
      Axlsx::sanitize(@shared_xml_string)
      str << ('<?xml version="1.0" encoding="UTF-8"?><sst xmlns="' << XML_NS << '"')
      str << (' count="' << @count.to_s << '" uniqueCount="' << unique_count.to_s << '"')
      str << (' xml:space="' << xml_space.to_s << '">' << @shared_xml_string << '</sst>')
    end
Creates a new Shared Strings Table agains an array of cells
 @param [Array] cells This is an array of all of the cells in the workbook
 @param [Symbol] xml_space The xml:space behavior for the shared string table.
 Serializes the object
 @param [String] str
 @return [String]