Can stretch params of type Array and Hash

This PR enables params that are of type Array and Hash.

The behaviour is to 'stretch' these params when calculating the signature.
This is the same behavior as OAuth gem (https://github.com/pelle/oauth).

Array params gets copied with the same key

    params = { "list" => ["a", "b", "c"] }
    # signature_params are [ ["list", "a"], ["list", "b"], ["list", "c"] ]

Hash params use bracket notiation

    params = { "hash" => { "a" => "1", "b" => "2" } }
    # signature_params are [ ["hash[a]", "1"], ["hash[b]", "2"] ]