Inspect both index and created keys for errored documents

For some reason, when migrating from an unaliased "metasearch",
documents come back with a "created" => key, rather than an "indexed"
key.

I have no idea why. But this stops the rake task from erroring and
allows the aliasing to complete successfully.
Merge pull request #289 from alphagov/allow-inspection-of-created-items

Inspect both index and created keys for errored documents
Add pry
SearchParameterParser is aware of field type
Parsed search parameter hash contains typed values
Parsed params contains filter object
CMA case opened_date should be indexed

In order to filter on this field it needs to be indexed, otherwise
elasticsearch silently returns zero results when trying to do the
filter.
Allow date filtering using only from or to

It should be possible to provide only one of the bounds for filtering
rather than having to give both.

Introduces a lightweight null date object so that we can call `iso8601`
on objects the parameter parser gives back without having to do `nil`
checking first.