allow requests with lists of strings as arguments (e.g search filter)

[powo]

If the kwargs of a request have values that are e.g list of strings, it resulted in an error because 'str' object has no attribute 'to_xml' e.g:

    SortBy=['State', 'Age'],
    OrderBy=['Up', 'Down'],

added support for this kind of arguments

[ewsterrenburg]

Thanks @powo