Type coercion

subtype 'HTTP::Header'
    => as 'Object'
    => where { $_->isa('HTTP::Headers') };

coerce 'HTTP::Header'
    => from 'ArrayRef'
        => via { HTTP::Headers->new( @{ $_ } ) }
    => from 'HashRef'
        => via { HTTP::Headers->new( %{ $_ } ) };