Switch from YAML to JSON for our serialization format.

This gets us around all of the issues with safe_yaml and the YAML
object serialization bugs.
Remove the Hash/Mash hack; let rsolr-ext do what it wants.
Add spec for DatasetsHelper.
Fix the info-query-error cassette.
Don't worry about coverage on RDF errors I don't know how to cause.
Fix strong parameters for ActiveAdmin pages.
Fix Resque failure spec to actually call code.
Add Document.find_by_shasum{,_with_fulltext}.

This mirrors the interface in ActiveRecord, which returns `nil`
rather than throwing an exception when a document cannot be found.
Take the code back to 100% coverage.