Generate an HTTP response in Ruby

It's actually not that hard to roll your own fake responses directly with Net::HTTP. Here's a simple 200 OK with a cookie header:

def fake_response
  net_http_resp = Net::HTTPResponse.new(1.0, 200, "OK")
  net_http_resp.add_field 'Set-Cookie', 'Monster'
  RestClient::Response.create("Body goes here", net_http_resp, nil)
end

Since few of us are using raw Net::HTTP anymore, the (optional) last line wraps it up as a RestClient::Response, which can then be stubbed into RestClient:

stub(RestClient).post(anything) { fake_response }

I would start with FakeWeb and see if that meets your needs. If it doesn't you can probably gut whatever you need out of the internals and create your own solution.