Add a function to assert headers

[iamjarvo]

No description provided.

[brjann]

I too would appreciate this functionality.

[glenjamin]

I'd take a PR for this if it isn't already part of peridot

[danielcompton]

I think this is correct, though I haven't done a super deep dive on it. Here for anyone who wants it, eventually I'll add this to kerodon, or someone else can make a PR?

(defmacro header? [header-name expected]
  `(ktest/validate =
                   (comp #(ring.util.response/get-header % ~header-name) :response)
                   ~expected
                   (~'header? ~header-name ~expected)))