Chore: Update gitignore to latest version
Bugfix: Don't use instance variables for storing the headers

Rails, unbeknownst to us, doesn't create a new instance of the constraint on
every request, therefore if we use instance variables for memoization, they
stick around for the next request.  Due to this, once the first request goes
through, whatever the result of the accept header verification _was_, it
_continues_ to be for each subsequent request.

This change removes any usages of instance variables in favor of local variables
inside `matches?`.
Chore: Move RescuableResource to HumanError

We now need the HumanError configuration in this thing.  Because it is
_extremely_ coupled to `human_error`, I'm just going to move it into there.