LegendGraphic doesn't show on layers that use "GetMap-url" parameter

[WCarlsson]

On layers that use different url:s, see #345, for GetCapabilities and other requests the legendGraphic doesn´t show.

This seems to be because the GetLegendGraphic uses the wrong url for the request, for example:
Url: https://resource.sgu.se/service/wms/130/jordarter-25-100-tusen-fastmark
GetMap-url: https://maps3.sgu.se/geoserver/jord/ows
GetMap and GetFeatureInfo uses the GetMap url but getLegendGraphic uses the normal url.

Steps to reproduce the behavior:

1. Set up a layer with different url:s
2. Add layer to a map
3. Expand to see legendGraphic
4. See error

Expected behavior
Legend Graphic should show as intended

[Hallbergs]

@WCarlsson would be great if you could make sure that the fix is working. (You can do the test when the release candidate for 3.10 is out).