a method to set an individual query parameter without removing the rest or keeping the original value

```py
from furl import furl

def url() -> furl:
return furl("http://foo.com/?a=1&b=2")

print(url().set({"a": 3})) # http://foo.com/?a=3
print(url().add({"a": 3})) # http://foo.com/?a=1&b=2&a=3

# i would like to be able do this in one method:
print(url().remove(["a"]).add({"a": 3})) # http://foo.com/?b=2&a=3
```

the current behavior of `add` where it supports multiple query params with the same value seems like an edge case imo