Fix error refcount bug

Added a missing ref in httpcli.c. Also added a test to avoid
regression

src/core/lib/http/httpcli.c

@@ -105,7 +105,7 @@ static void finish(grpc_exec_ctx *exec_ctx, internal_request *req,
                    grpc_error *error) {
   grpc_polling_entity_del_from_pollset_set(exec_ctx, req->pollent,
                                            req->context->pollset_set);
-  grpc_closure_sched(exec_ctx, req->on_done, error);
+  grpc_closure_sched(exec_ctx, req->on_done, GRPC_ERROR_REF(error));
   grpc_http_parser_destroy(&req->parser);
   if (req->addresses != NULL) {
     grpc_resolved_addresses_destroy(req->addresses);

test/cpp/client/credentials_test.cc

@@ -50,6 +50,10 @@ TEST_F(CredentialsTest, InvalidGoogleRefreshToken) {
   EXPECT_EQ(static_cast<CallCredentials*>(nullptr), bad1.get());
 }
 
+TEST_F(CredentialsTest, DefaultCredentials) {
+  auto creds = GoogleDefaultCredentials();
+}
+
 }  // namespace testing
 }  // namespace grpc