case_officer_code modifies Authority url param

[adrianshort]

How to reproduce:

auth = UKPlanningScraper::Authority.named("X") # Northgate authorities only
apps1 = auth.case_officer_code("123").decided_days(7).scrape
apps2 = auth.decided_days(7).scrape

This fails because the url param of Authority has been modified so the apps2 call to scrape fails because it's trying to scrape from the case officer page rather than the general search page.

The url param needs to stay constant throughout the lifetime of the object so scrape will always be requesting the right URL.