To be pedantic, in Ruby, the fact that an object is of class String is not sufficient evidence that a method has not been overloaded (they can be overloaded via the objects eigenclass), though you're probably right.
Even if it did (and it doesn't, I checked), you're probably still better off doing it explicitly since it's easier to audit, less likely to differ across implementations, and less error prone (e.g. swap the order of the comparison and you're back to plain old String#==).