Fix Bug: retry scrape will lost POST requestData

[Shinku-Chen]

when retry scrape requestData will loss in http.NewRequest

so Seek requestData before scrape.NewRequest

req.ContentLength  always 0 and

if req.GetBody != nil && req.ContentLength == 0 {
			req.Body = NoBody
			req.GetBody = func() (io.ReadCloser, error) { return NoBody, nil }
		}