code: func HandleString(f func(s string, ctx *goproxy.ProxyCtx) string) goproxy.RespHandler {
  	return HandleStringReader(func(r io.Reader, ctx *goproxy.ProxyCtx) io.Reader {
  		b, err := ioutil.ReadAll(r)
  		if err != nil {
  			ctx.Warnf("Cannot read string from resp body: %v", err)
  			return r
  		}
  		return bytes.NewBufferString(f(string(b), ctx))
  	})
  }
docs: // HandleString will receive a function that filters a string, and will convert the
  // request body to a utf8 string, according to the charset specified in the Content-Type
  // header.
  // guessing Html charset encoding from the <META> tags is not yet implemented.