golf.diff

diff --git a/lib/Mojo/Headers.pm b/lib/Mojo/Headers.pm
index 320d4a4..cdac39b 100644
--- a/lib/Mojo/Headers.pm
+++ b/lib/Mojo/Headers.pm
@@ -20,9 +20,14 @@ my %NORMALCASE = map { lc() => $_ } (
   qw(Upgrade User-Agent Vary WWW-Authenticate)
 );
 for my $header (values %NORMALCASE) {
-  my $name = lc $header;
+  my $name = my $key = lc $header;
   $name =~ y/-/_/;
-  monkey_patch __PACKAGE__, $name, sub { shift->header($header => @_) };
+  monkey_patch __PACKAGE__, $name, sub {
+    my $self = shift;
+    $self->{headers}{$key} = [@_] and return $self if @_;
+    return undef unless my $headers = $self->{headers}{$key};
+    return join ', ', @$headers;
+  };
 }
 
 sub add {