There is a slight difference in the allowed types between String#initialize and String#new.

The current signature focuses on String#initialize, and includes cases where String.new would fail.

String.new(capacity: nil)
#=> no implicit conversion from nil to integer (TypeError)

There's an option to separate the signatures for String.new and String#initialize, but it's hard to see the benefit of allowing capacity and encoding to be nil.

In this revision, I've changed to a signature that only works with String.new.
I believe this is simpler and more practical.