Uppercase and lowercase generated test function collide

Hello,

If you write tests like this, the generated function names will collide (`_a_expects`):

```rust
#[test_case('a')]
#[test_case('A')]
fn test (_: char) {
}
```
I worked around it by adding an extra argument in front:
```rust
#[test_case(1, 'a')]
#[test_case(2, 'A')]
fn test (_: u32, _: char) {
}
```

Maybe it would be easy to suffix the index of the test case to the generated name (`_a_expects_{}`).

Thanks.