func IsUnknownCharset(err error) bool {
	return errors.As(err, new(UnknownCharsetError))
}
IsUnknownCharset returns a boolean indicating whether the error is known to
report that the charset advertised by the entity is unknown.
package message
import (
	"errors"
	"fmt"
	"io"
	"mime"
	"strings"
)
type UnknownCharsetError struct {
	e error
}
func (u UnknownCharsetError) Unwrap() error { return u.e }
func (u UnknownCharsetError) Error() string {
	return "unknown charset: " + u.e.Error()
}