Make sure $download_list is defined before concatenating strings.
Merge pull request #2816 from dannyvankooten/fix/undefined-var-download-list

Make sure $download_list is defined before concatenating strings.
Add @cklosowski to contributors list. How had this not been done before?
Perform HTML5 validation before sending off AJAX request.
Merge branch 'master' of github.com:easydigitaldownloads/Easy-Digital-Downloads

# By Danny van Kooten (1) and Pippin Williamson (1)
# Via Pippin Williamson
* 'master' of github.com:easydigitaldownloads/Easy-Digital-Downloads:
  Add @cklosowski to contributors list. How had this not been done before?
  Make sure $download_list is defined before concatenating strings.