net/http/fcgi: fix test

net/http/fcgi: fix test

Currently the test fails if run more than once:

$ go test -v -run=TestChildServeCleansUp -cpu=1,1 net/http/fcgi
=== RUN TestChildServeCleansUp
--- PASS: TestChildServeCleansUp (0.00s)
=== RUN TestChildServeCleansUp
fatal error: all goroutines are asleep - deadlock!

The problem is that the writer mutates test input data,
so it is wrong on the second execution.

Change-Id: I4ca54dd2926c6986b2908023ac65e5e65630ed26
Reviewed-on: https://go-review.googlesource.com/6383
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>