Dwithin spatial_query bug - don't check units and always puts degrees

When a CSW GetRecords request is run against a PostgreSQL backend and uses Dwithin spatial_query, the query that is generated to PostgreSQL is in "degree" units instead of "metres" as requested

**Problem Description**
When querying the PYCSW with the dwithin spatial_query, for example:

ows:BoundingBox

23.7342660 37.974720

50

When pycsw translates the query, it ignores the Distance units="meters"
From:
https://github.com/geopython/pycsw/blob/9b9afc9a4b6dbc4ba68d071262fdcce8eaa039b9/pycsw/ogc/fes/fes2.py#L375

We need to change the generated query from:
`WHERE st_dwithin(wkb_geometry, st_geomfromtext('POINT(23.7342660 37.974720)',4326), 50.000000) = 'true'`
To:
`WHERE st_dwithin(wkb_geometry::geography, st_geomfromtext('POINT(23.7342660 37.974720)',4326)::geography, 50.000000) = 'true'`