Rack spec requires mutable headers

The rack spec requires the header object to be an unfrozen hash.
c8e9822183/SPEC.rdoc (L240)

Rack::ETag was buffering and making a copy of the response,
so the freeze was not effective anyway.

Plus we are freezing the hash too early, preventing middlewares
from modifying it. It causes crash with gems like rack-livereload.
I started having crashes on some pages (like the internal
http://localhost:3000/rails/info/routes) because of rack-livereload
hitting the frozen hash after the rack 3 upgrade.

Also we're not consistent with the protection. We're not preventing
users from adding cookies. The cookie jar is already flushed,
therefore it doesn't try to change the headers and never triggers the
frozen hash error.

actionpack/lib/action_dispatch/http/response.rb

@@ -462,7 +462,6 @@ def before_sending
       # our last chance.
       commit! unless committed?
 
-      @headers.freeze
       @request.commit_cookie_jar! unless committed?
     end
 

actionpack/test/dispatch/live_response_test.rb

@@ -86,36 +86,6 @@ def test_content_length_is_removed
         @response.stream.write "omg"
         assert_nil @response.headers["Content-Length"]
       end
-
-      def test_headers_cannot_be_written_after_web_server_reads
-        @response.stream.write "omg"
-        latch = Concurrent::CountDownLatch.new
-
-        t = Thread.new {
-          @response.each do
-            latch.count_down
-          end
-        }
-
-        latch.wait
-        assert_predicate @response.headers, :frozen?
-        assert_raises(FrozenError) do
-          @response.headers["Content-Length"] = "zomg"
-        end
-
-        @response.stream.close
-        t.join
-      end
-
-      def test_headers_cannot_be_written_after_close
-        @response.stream.close
-        # we can add data until it's actually written, which happens on `each`
-        @response.each { |x| }
-
-        assert_raises(FrozenError) do
-          @response.headers["Content-Length"] = "zomg"
-        end
-      end
     end
   end
 end