Added a regression test for expiry param equal to 'None'.

For a not expiring item.
Discard `None` params in `LaterPayClient._get_web_url()`

We don't want to create urls containing "param=None" bits
if a param is optional.
Merge pull request #19 from laterpay/feature/fix-expiry-param-as-None-in-url

Discard `None` params in `LaterPayClient._get_web_url()`