feat: add response.ToJson

[qiankunli]

usage example

c := NewHttpClient()
res, err := c.Get("http://httpbin.org/user-agent")

if err != nil {
  t.Error(err)
}
type Result struct {
   UserAgent string `json:"user-agent"`
}
result := &Result{}
err = res.ToJson(result)
if err != nil {
  t.Error(err)
}