Normalize trigram query and document as Arel nodes

We need to normalize the Trigram's query and document, making sure they
are Arel nodes, not strings so that the
`Arel::Nodes::InfixOperation.new` correctly puts them together with the
'%' operator. Without this change, they are incorrectly escaped as:

```sql
(
  ('unaccent(coalesce("pg_search_documents"."content"::text, ''''))')
  %
  'unaccent(Cambonator Doppelbock)'
)
```

The correct escaping is:

```sql
(
  (unaccent(coalesce("pg_search_documents"."content"::text, '')))
  %
  unaccent('Cambonator Doppelbock')
)

```