Signature change of ddt.idata in 1.4.3 breaks downstream uses

[jakelishman]

In #92, the signature of idata was changed from idata(values) to idata(values, index_len), with no default handling for the latter argument. This means that all current uses of idata(values) are now broken on upgrade to 1.4.3, and there's no compatible calling method for versions 1.4.2 and 1.4.3.

I'm not too familiar with your internals, but glancing through PR #92, it looks like it could be handled safely without affecting that PR by changing the signature to something like:

def idata(values, index_len=None):
    if index_len is None:
        # Avoid accidentally consuming a one-time-use iterable.
        values = tuple(values)
        index_len = len(str(len(values)))
    # ... continue as normal ...

If so, I'd be happy to make the PR.