Allow passing a Pathname object to the path argument

[yujideveloper]

I got the below error when passing Pathname object to PDFKit#to_file in PDFKit v0.8.7.2.

TypeError:
  no implicit conversion of Pathname into String
# /bundle/ruby/2.7.0/gems/pdfkit-0.8.7.2/lib/pdfkit/pdfkit.rb:71:in `popen'
# /bundle/ruby/2.7.0/gems/pdfkit-0.8.7.2/lib/pdfkit/pdfkit.rb:71:in `to_pdf'
# /bundle/ruby/2.7.0/gems/pdfkit-0.8.7.2/lib/pdfkit/pdfkit.rb:84:in `to_file'

I think this error introduced in v0.8.7.2(#519).

[github-actions]

This pull request has been marked as stale and will be automatically closed.

[yujideveloper]

@serene
Thank you for your review. Could you merge this PR?