Don't require rspec/autorun

This is unneccessary unless you plan to run spec files directly instead
of via rake or rspec and also creates issues with test runners like
zeus because it adds an at_exit hook to execute specs.
Pass through options on as_json override...

... Otherwise much of the utility of as_json is lost.
Whitelist publicly visible attributes

Safer than blacklisting the internal ones
Merge pull request #16 from alphagov/minor-tweaks

Minor tweaks