fixes #399 TypeError when parsing login response, fixes #397 throw when cert is null

[sultanhq]

There has been an issue in the library that means it cannot handle multiple anonymous certificates for signing/encrypting.

The implementation to resolve this was to add an exclude attribute which will allow the extractor to get keys which do not include specified attributes.

Also this handling of not finding certificates would not provide a suitable error (fix provided from another PR (still awaiting acceptance))

fixes #399 TypeError when parsing login response, fixes #397 throw when cert is null.