proxy: fix TestReverseProxy failure on go tip (#1360)
proxy: fix TestReverseProxy failure on go tip (#1360)

Because of this commit(6e36811c37399d60cbce587b7c48e611009c5aec) on go tip,
it will probe the request's body to determine whether to use chunked transfer
encoding which trailers depend on it.
So we just offer a non empty body to make trailers work.

fix issue #1359

Signed-off-by: Tw <9badae619f85d6141021d55221ac5bc6f13849e6@gmail.com>
pyed/caddy,elcore/caddy,captncraig/caddy,devangels/caddy,captncraig/caddy,devangels/caddy,dusty/caddy,tobya/caddy,joshix/caddy,captncraig/caddy,tobya/caddy,pyed/caddy,dusty/caddy,elcore/caddy,tobya/caddy,dusty/caddy,joshix/caddy,dusty/caddy,joshix/caddy,pyed/caddy,pyed/caddy,cez81/caddy,cez81/caddy,yukimochi/caddy,yukimochi/caddy,devangels/caddy,yukimochi/caddy,elcore/caddy,tobya/caddy,cez81/caddy