type guessing a column of zeros (0) fails with exception

[samhatchett]

due to this line:
https://github.com/okfn/messytables/blob/master/messytables/types.py#L291

... if not cell.value evaluates to true when cell.value === 0 and so no guesses are stored for that column. Which means that the guess list is empty and that causes an exception later on.

I see that this whole code chunk is removed in other branches and the project seems to be in a kind of flux, so not sure if this is already fixed and just waiting to be merged/released, but thought it worth documenting here.