http: parametrize serialize_flatten to choose array representation

Commit c7eab5d04ba09d66d2c7e9ba53efffbe9048af30 changed the way
serialize_flatten treats Python lists, from:

  array[0]=val1&array[1]=val2&array[2]=val3

to:

  array[]=val1&array[]=val2&array[]=val3

This helped some services (like UserVoice), but broke others (like
Mailchimp). Seems that the only way is to provide a parameter to
control the behaviour.