allow us to not specify headers

set headers aren't always needed. this allows us to remove the default added headers.

nginx::resource::location {
    'somevhost-somelocation':
      ensure   => present,
      vhost    => 'somevhost',
      location => '~ ^/somedir/stuff.*$',
      proxy    => 'http://10.0.0.1:80',
      proxy_set_header => '';
}