Clear and merge test response headers instead of assigning.

Rails 5 removes the attr_writer for this method, but clearing the hash
and writing to it is essentially equivalent to assigning it.

Author: Sam Phippen

lib/rspec/rails/matchers/have_http_status.rb

@@ -43,7 +43,8 @@ def as_test_response(obj)
             # Capybara or catch `NameError`s for the undefined constants
             obj = ActionDispatch::Response.new.tap do |resp|
               resp.status  = obj.status_code
-              resp.headers = obj.response_headers
+              resp.headers.clear
+              resp.headers.merge!(obj.response_headers)
               resp.body    = obj.body
             end
             ::ActionDispatch::TestResponse.from_response(obj)