Add special_instructions to the permitted checkout attributes

* When special instructions are enabled on checkout, this attribute
  was being ignored.
* No specs are included with this because I could not find any specs
  related to permitted attributes, so I didn't have anything to use
  for reference. I assume that they are not present because the risk
  of a regression is low. However, I would be happy to add a spec
  given some direction.

Fixes #3784