Fix client to work with cert_chain_uri, "is (not) None" fixes

[kuba]

Demo was recently dirty-fixed to include a chain. This uncovered a couple of bugs.

• @schoen hit it yesterday when working on renewer
• it is also a blocker for dvsni support for nginx #387

We should write unittests for this module ASAP! Any kind volunteers? :)

Also, please use if foo is None or if foo is not None and not just if foo. Provided that foo can never be None, if foo should only be used if foo is a string or a container (sequence, dict, collection, etc.) and one wants to check if it's non-empty.

[coveralls]

Coverage remained the same at 92.32% when pulling a3c8753 on kuba:client-fetch_chain-bugs into ae31b81 on letsencrypt:master.

[jdkasten]

Thanks!