Catch collections not collection in CQL filters

[vincentsarago]

This PR does:

• add tests for collections search
• make sure collections and not collection get translated to CQL.

closes #42

[vincentsarago]

🤷‍♂️ I don't get why the tests pass. Locally running

SELECT * from search('{"collections":["pgstac-test-collection"],"fields":{"include":["id"]}, "limit": 1}')

returns {"next": null, "prev": null, "type": "FeatureCollection", "context": {"limit": 1, "matched": 0, "returned": 0}, "features": null}

while the incorrect query

SELECT * from search('{"collection":["pgstac-test-collection"],"fields":{"include":["id"]}, "limit": 1}');

returns {"next": "20200307aC0870130w361200", "prev": null, "type": "FeatureCollection", "context": {"limit": 1, "matched": 353, "returned": 1}, "features": [{"id": "20200307aC0870130w361200"}]} which is the expected results for the previous command 🤷‍♂️