Selection of boundaries for the default implementation of is_case

[r-bk]

@rutrum Hi!
Firstly, thanks for such a useful crate!

I have found the following example which IMO has a not intuitive result.

#[test]
fn test_upper_snake() {
    use convert_case::{Case, Casing};
    assert!("UPPER_CASE_WITH_DIGIT1".is_case(Case::UpperSnake));
}

This test fails, even though UpperSnake case has only a single boundary _ and the string is in capital letters.

This happens because the implementation of is_case initializes the StateConverter with Boundary::defaults() which includes UpperDigit boundary.

Is this by design, or a bug?

If a bug, would the correct implementation of is_case be something like this?

fn is_case(&self, case: Case) -> bool {
    &self.from_case(case).to_case(case) == self
}