Setting multiple query params for the same key and resultant url format

Hi,

I a bit confused how to create an url with multiple query params with the same key, (I was reading an old issue #69)

this one works:
```
f = furl("https://example.com").add(path=["path1", "path2"])
f.args.setlist('foobar', ["one", "two"])
print(f.url)
# gives https://example.com/path1/path2?foobar=one&foobar=two
```
but this too works
```
f = furl("https://example.com").add(path=["path1", "path2"])
f.args["foobar"] = ["one", "two"]
print(f.url)
# gives https://example.com/path1/path2?foobar=one&foobar=two
```

What is the right way to setting multiple query params for the same key?

About the resultant url, can I be sure it that the multiple query params for the same key will be always in the format `foobar=one&foobar=two` and not in the format `foobar=one,two` ?

Thanks for the help.