code: func InvalidContentType(value string, allowed []string) *Validation {
  	values := make([]interface{}, 0, len(allowed))
  	for _, v := range allowed {
  		values = append(values, v)
  	}
  	return &Validation{
  		code:    http.StatusUnsupportedMediaType,
  		Name:    "Content-Type",
  		In:      "header",
  		Value:   value,
  		Values:  values,
  		message: fmt.Sprintf(contentTypeFail, value, allowed),
  	}
  }
docs: // InvalidContentType error for an invalid content type