Stop retriable request from adding a non-nil Body (#726)

When azure is responding with an error the retriable request that is generated ends up getting a non nil noOpcloser added to it with a reader interface. Ultimately this ends up sending a GET request to Azure that has a post body. As the http spec says that the body SHOULD be ignored this is not a critical error, but for people using custom transports this is problematic. It's also sending unessecary bytes.

autorest/retriablerequest_1.7.go

@@ -33,7 +33,7 @@ type RetriableRequest struct {
 func (rr *RetriableRequest) Prepare() (err error) {
 	// preserve the request body; this is to support retry logic as
 	// the underlying transport will always close the reqeust body
-	if rr.req.Body != nil {
+	if rr.req.Body != nil && rr.req.Body != http.NoBody {
 		if rr.br != nil {
 			_, err = rr.br.Seek(0, 0 /*io.SeekStart*/)
 			rr.req.Body = io.NopCloser(rr.br)

autorest/retriablerequest_1.8.go

@@ -34,7 +34,7 @@ type RetriableRequest struct {
 func (rr *RetriableRequest) Prepare() (err error) {
 	// preserve the request body; this is to support retry logic as
 	// the underlying transport will always close the reqeust body
-	if rr.req.Body != nil {
+	if rr.req.Body != nil && rr.req.Body != http.NoBody {
 		if rr.rc != nil {
 			rr.req.Body = rr.rc
 		} else if rr.br != nil {