Incorrect cleanup of rules vector in Rule::operator=

[aldoshkind]

Hello! I suppose there are incorrect cleanup of rules vector in Rule::operator=() here: https://github.com/medooze/sdp-cpp/blob/4bf23de4cf6ade65fedb68a8c8a5baf4bd6a3e6d/src/abnf/Rule.cpp#L50

Shouldn't there be clear() insted of empty()? In any case calling empty() without using it's return value makes no sense.