treat undef headers as the empty string

fixes the issue described at http://www.jwz.org/blog/2015/09/more-lwp-bit-rot/

Author: karenetheridge

Changes

@@ -1,5 +1,7 @@
 Revision history for HTTP-Message
 
+    - fix an undefined value warning in HTTP::Headers::as_string
+
 6.10   2015-07-19
 
     - fix uses of qr/.../m in tests that do not work in 5.8.x

lib/HTTP/Headers.pm

@@ -263,6 +263,7 @@ sub as_string
 	my $vals = $self->{$key};
 	if ( ref($vals) eq 'ARRAY' ) {
 	    for my $val (@$vals) {
+		$val = '' if not defined $val;
 		my $field = $standard_case{$key} || $self->{'::std_case'}{$key} || $key;
 		$field =~ s/^://;
 		if ( index($val, "\n") >= 0 ) {
@@ -272,6 +273,7 @@ sub as_string
 	    }
 	}
 	else {
+	    $vals = '' if not defined $vals;
 	    my $field = $standard_case{$key} || $self->{'::std_case'}{$key} || $key;
 	    $field =~ s/^://;
 	    if ( index($vals, "\n") >= 0 ) {