def initialize(cells, xml_space=:preserve)
      @index = 0
      @xml_space = xml_space
      @unique_cells = {}
      @shared_xml_string = ""
      shareable_cells = cells.flatten.select{ |cell| cell.plain_string? || cell.contains_rich_text? }
      @count = shareable_cells.size
      resolve(shareable_cells)
    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.