Search params are not deleted

[JohannesDoberer]

Fixes #2491
How to test:

• Please test with useHashrouting = true/false

LuigiClient:

searchParams:
- LuigiClient.addCoreSearchParams({q:'test', luigi:'rocks', tets:'tets'});
- LuigiClient.addCoreSearchParams({q:undefined})
nodeParams:
LuigiClient.addNodeParams({q:'test', luigi:'rocks'});
LuigiClient.addNodeParams({q:undefined, luigi:'rocks'});

Luigi Core

searchParams:
Luigi.routing().addSearchParams({test:'tets'})
Luigi.routing().addSearchParams({test:undefined})

nodeParams:
Luigi.routing().addNodeParams({test:'tets'})
Luigi.routing().addNodeParams({test:undefined})

[wdoberschuetz]

LGTM :).