Only retry idempotent requests. (#80)

This also reverts a change to send_body that was originally added to
return the number of bytes written. It's no longer needed now that we
check the size of the reader in advance.

Fixes #76.

src/test/timeout.rs

@@ -1,5 +1,3 @@
-
-use crate::test;
 use std::io::{self, BufRead, BufReader, Read, Write};
 use std::net::TcpStream;
 use std::thread;