ensure string is passed to truncate since errors with integers

[hudajkhan]

Relates to having tests pass with Blacklight 8 (specially 8.3).

For 8.3, the following test is failing:
rspec ./spec/views/spotlight/sir_trevor/blocks/_solr_documents_features_block.html.erb_spec.rb:48 # spotlight/sir_trevor/blocks/_solr_documents_features_block.html.erb without a primary caption falls back to the regular document title for the caption

The reason appears to be when the truncate method is passed an integer, e.g. truncate(1, length:95), it throws an error. However, truncate("1", length:95) will work just fine. Adding a "to_s" ensures the caption text truncation does not throw an error.

This test should still pass in Blacklight 7.

How to test with Blacklight 8: Generate test app (.internal_test_app). Update the internal test app's Gemfile to use Blacklight 8.3. Run bundle install for the internal test app. Then run rspec ./spec/views/spotlight/sir_trevor/blocks/_solr_documents_features_block.html.erb_spec.rb:48.

Relates to #3047 and this comment which includes a checklist of tests to fix: #3047 (comment)

[jcoyne]

What causes the caption text to be an integer in the first place?

[cbeer]

This might be an artifact of the test setup (falling back on an integer id field), but it's plausible (if not likely) that this could happen in reality.