code: func (F *Frisby) ExpectHeader(key, value string) *Frisby {
  	Global.NumAsserts++
  	chk_val := F.Resp.Header.Get(key)
  	if chk_val == "" {
  		err_str := fmt.Sprintf("Expected Header %q, but it was missing", key)
  		F.AddError(err_str)
  	} else if chk_val != value {
  		err_str := fmt.Sprintf("Expected Header %q to be %q, but got %q", key, value, chk_val)
  		F.AddError(err_str)
  	}
  	return F
  }
docs: // Checks for header and if values match