net/http/fcgi: Request.Body should always be non-nil
net/http/fcgi: Request.Body should always be non-nil

Found this inconsistency from net/http's Server while
debugging Issue 4183

Unfortunately this package lacks testing around this,
or most of child.go. :/

R=golang-dev, adg
CC=golang-dev
https://codereview.appspot.com/7735046

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