Update fragment actions for top level fragment API

Allow passing a falsy value as the collection and construct the appropriate URL.

Would be better if the arguments were reversed: (fragment, collection = null) but this would be quite a disruptive change. Worth it?

Closes #54

assigned to @malexsan1

assigned to @bogdan

I think this fits the bill for now. The API would indeed be cleaner if the arguments were reversed, but one step at a time. LGTM

assigned to @jure

merged

mentioned in commit 6482f315