[ruby/net-http] Fix tests after change to allow HEAD to send Accept-Encoding

https://github.com/ruby/net-http/commit/ca7bb8c51c

test/net/http/test_http_request.rb

@@ -46,8 +46,9 @@ class HTTPRequestTest < Test::Unit::TestCase
     assert_not_predicate req, :response_body_permitted?
 
     expected = {
-      'accept'     => %w[*/*],
-      'user-agent' => %w[Ruby],
+      'accept'          => %w[*/*],
+      "accept-encoding" => %w[gzip;q=1.0,deflate;q=0.6,identity;q=0.3],
+      'user-agent'      => %w[Ruby],
     }
 
     assert_equal expected, req.to_hash.tap{_1.delete 'accept-encoding'}