Header offset guessing does not work with type guessing

Type guessing is done before applying the offset in https://github.com/okfn/dataconverters/blob/master/dataconverters/commas.py#L56, which is wrong. 

This problem has not been noticed because of https://github.com/okfn/messytables/issues/52. 

My fix is probably the worst possible but a proper fix would require a refactoring of the code in `commas.py` and `xls.py`. IMHO, doing the type guessing before the offset application makes it really difficult to understand and debug. Have a look at https://github.com/okfn/ckanext-datastorer/blob/master/ckanext/datastorer/tasks.py#L105.