Fix no implicit conversion error when datetime cell with hyperlink

[muraaano]

Summary

I found a bug and this PR to fix that.

Bug

• If there is a cell as datetime and set to hyperlink, float value will be passed to Roo::Link.new. It cause type error from float to string.
• How to produce bug:
  • execute below in irb

  xlsx = Roo::Spreadsheet.open(Rails.root.join("datetime_with_link.xlsx").to_s)
  xlsx.info
  
  TypeError (no implicit conversion of Float into String)
  

  • datetime_with_link.xlsx is here
• Environment:
  • ruby '2.6.6'
  • roo (2.8.2)

Other Information

I appreciate if this patch make gem better. Feel free to comment better implementation or anythingelse.
Thanks.