my $self = shift;
    return sort {
        ($header_order{$a} || 999) <=> ($header_order{$b} || 999) ||
         $a cmp $b
    } keys %$self
}


sub header_field_names {
    my $self = shift;
    return map $standard_case{$_} || $_, $self->_sorted_field_names
	if wantarray;
    return keys %$self;
}


sub scan
{
    my($self, $sub) = @_;
    my $key;
    foreach $key ($self->_sorted_field_names) {
        next if $key =~ /^_/;
	my $vals = $self->{$key};
	if (ref($vals) eq 'ARRAY') {
	    my $val;
	    for $val (@$vals) {
		&$sub($standard_case{$key} || $key, $val);
	    }
	}
	else {
	    &$sub($standard_case{$key} || $key, $vals);
	}
    }
}


sub as_string
{
    my($self, $endl) = @_;
    $endl = "\n" unless defined $endl;

    my @result = ();
    $self->scan(sub {
	my($field, $val) = @_;
	$field =~ s/^://;
	if ($val =~ /\n/) {
	    # must handle header values with embedded newlines with care
	    $val =~ s/\s+$//;          # trailing newlines and space must go
	    $val =~ s/\n\n+/\n/g;      # no empty lines
	    $val =~ s/\n([^\040\t])/\n $1/g;  # intial space for continuation
	    $val =~ s/\n/$endl/g;      # substitute with requested line ending
	}
	push(@result, "$field: $val");
    });

    join($endl, @result, '');
}


if (eval { require Storable; 1 }) {
    *clone = \&Storable::dclone;
} else {
    *clone = sub {
	my $self = shift;
	my $clone = new HTTP::Headers;
	$self->scan(sub { $clone->push_header(@_);} );
	$clone;
    };
}


sub _date_header
{
    require HTTP::Date;
    my($self, $header, $time) = @_;
    my($old) = $self->_header($header);
    if (defined $time) {
	$self->_header($header, HTTP::Date::time2str($time));
    }
    $old =~ s/;.*// if defined($old);
    HTTP::Date::str2time($old);
}


sub date                { shift->_date_header('Date',                @_); }
sub expires             { shift->_date_header('Expires',             @_); }
sub if_modified_since   { shift->_date_header('If-Modified-Since',   @_); }
sub if_unmodified_since { shift->_date_header('If-Unmodified-Since', @_); }
sub last_modified       { shift->_date_header('Last-Modified',       @_); }

# This is used as a private LWP extension.  The Client-Date header is
# added as a timestamp to a response when it has been received.
sub client_date         { shift->_date_header('Client-Date',         @_); }

# The retry_after field is dual format (can also be a expressed as
# number of seconds from now), so we don't provide an easy way to
# access it until we have know how both these interfaces can be
# addressed.  One possibility is to return a negative value for
# relative seconds and a positive value for epoch based time values.
#sub retry_after       { shift->_date_header('Retry-After',       @_); }