Refactor spec test to address Blacklight::Document deprecation; DDR-1718.

Slight reorganization of code makes it look like more changed than actually did. Basic change was that subject for instance methods is now instantiated with a source hash which can be populated as needed. Deprecations addressed by replacing former subject[index_field] = 'foo' in before block with let(:source) { { index_field => 'foo' } }.