free-text search with unquoted, space-separated terms does not work

If you pass `"q": "word1 word2"` to a search, it will fail with a `tsquery syntax error`. If the phrase is quoted (e.g. `"q": "\"word1 word2\""), it works but it is a slightly different search.

To stay compliant with the OGC specification for the `q` parameter I think we should represent space-separated terms (without other operators) using the adjacency operator `<->`.