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 <tw19881113@gmail.com>