lib/rest: add support for setting trailers

lib/rest/rest.go

@@ -146,6 +146,7 @@ type Opts struct {
 	MultipartFileName     string       // ..name of the file for the attached file
 	Parameters            url.Values   // any parameters for the final URL
 	TransferEncoding      []string     // transfer encoding, set to "identity" to disable chunked encoding
+	Trailer               *http.Header // set the request trailer
 	Close                 bool         // set to close the connection after this transaction
 	NoRedirect            bool         // if this is set then the client won't follow redirects
 }
@@ -239,6 +240,9 @@ func (api *Client) Call(ctx context.Context, opts *Opts) (resp *http.Response, e
 	if len(opts.TransferEncoding) != 0 {
 		req.TransferEncoding = opts.TransferEncoding
 	}
+	if opts.Trailer != nil {
+		req.Trailer = *opts.Trailer
+	}
 	if opts.Close {
 		req.Close = true
 	}