fix: encode user agent

[maelle]

@salvafern I'm getting unrelated test failures like:

Is it possible that some layers changed types?!

emodnet.wfs::emodnet_get_layers(
    service = "seabed_habitats_individual_habitat_map_and_model_datasets",
    layers = c("dk003069", "dk003070"),
    simplify = TRUE
  )
#> Loading ISO 19139 XML schemas...
#> Loading ISO 19115 codelists...
#> ✔ WFS client created successfully
#> ℹ Service: "https://ows.emodnet-seabedhabitats.eu/geoserver/emodnet_open_maplibrary/wfs"
#> ℹ Version: "2.0.0"
#> Error in `emodnet.wfs::emodnet_get_layers()`:
#> ! Can't reduce layers when one is a data.frame
#> ℹ data.frame layer(s): "dk003069, dk003070"

^{Created on 2025-01-24 with reprex v2.1.1}

[maelle]

Or probably guess_layer_format() needs to be tweaked.

[maelle]

@salvafern eblondel/ows4R#138