* Updated SHA-256 functionality in the get_url module
  (fixes issue #4277)

* The sha256sum error message now displays the invalid
  destination checksum if it doesn't match the one that
  is specified in the arguments. This is useful for
  debugging purposes.

* Non-alphanumeric characters, including the infamous
  Unicode zero-width space, are removed from the sha256sum
  argument prior to the check.