Use `StringIO.new` instead of `""` #1284

Job	Run time
test-memory (head)	33s
steep-check (head)	31s
check-misc (head)	5s
test-ruby (head, master)	5m 24s
test-windows (head)	3m 13s
test-ruby (3.0, master)	5m 30s
ruby-versions / Generate Ruby versions	8s
test (2.5)	22s
test (2.6)	24s
test (2.7)	24s
test (3.0)	16s
test (3.1)	10s
test (3.2)	10s
test (3.3)	11s
test (head)	15s
	17m 36s

Provide feedback