Prevent bugs caused by multiple Close

See: https://github.com/golang/go/issues/19186
When the body isn't fully read and Close is called multiple times we put
the same buffer in the pool multiple times causing it to be reused by
multiple goroutines at the same time.

buffer/pool.go

@@ -252,6 +252,8 @@ func (r *readCloser) Close() error {
 	for _, buf := range r.bufs {
 		putBuf(buf)
 	}
+	// In case Close gets called multiple times.
+	r.bufs = nil
 
 	return nil
 }