undefined local variable or method `files' in cat method

[RevoHoffman]

Line 68 in /pdf-forms/blob/master/lib/pdf_forms/pdftk_wrapper.rb references the "files" array which is undefined.

You can dupe with:

pdftk = PdfForms.new('/usr/local/bin/pdftk')
pdftk.cat("file1.pdf","file2.pdf","output.pdf")

A quick look at the source indicates that "files" should be "input" instead. I'll try and fix and send a pull request, but wanted to report the issue first.