format_inner does not remove all trailing newlines

[amtoine]

Later we would need to fix the failing test, but nonetheless is a step forward!
I'm allright with a merge right now 😄

Originally posted by @AucaCoyan in #31 (comment)

the test here is the following

    #[test]
    fn remove_additional_lines() {
        let input = "let one = 1\n\n\n";
        let expected = "let one = 1\n";
        run_test(input, expected);
    }

i think it should become

    #[test]
    fn remove_additional_lines() {
        let input = "let one = 1\n\n\n";
        let expected = "let one = 1";
        run_test(input, expected);
    }

Note
the is no more newlines at the end of expected => this was moved out of the responsibility of format_inner in #31.

[AucaCoyan]

The PR #35 solves this issue 🎉