t_before breaks for date types

[ka7eh]

cql2 specs mentions instances used with temporal can be date or timestamp (https://docs.ogc.org/DRAFTS/21-065.html#_temporal_data_types_and_instances), but the following example (taken from https://github.com/opengeospatial/ogcapi-features/blob/master/cql2/standard/schema/examples/json/example20.json) fails with pygeofilter:

{
  "op": "t_before",
  "args": [
    { "property": "built" },
    { "date": "2015-01-01" }
  ]
}