Time can't be used as a Cell value

[timon]

Hi there,

I have stumbled upon an issue when rubyXL doesn't write the Time values:

require 'rubyXL'
workbook = RubyXL::Workbook.new
worksheet = workbook[0]
# observe the value being nil:
worksheet.add_cell(0, 0, Time.now) # =>  #<RubyXL::Cell(0,0): nil, datatype=nil, style_index=0>
# this works:
worksheet.add_cell(0, 1, DateTime.now) # => #<RubyXL::Cell(0,0): 44029.61175965089, datatype=nil, style_index=0>
workbook.write("out.xlsx")

The resulting file consists of one empty cell and one cell with value