Double Commas in #derive eats entire #derive

Input:
```rust
#[derive(Debug, ,Clone)]
pub enum Test {
    One,
    Two
}
```
(note the double commas after `Debug` above. The space is irrelevant in my testing)

Outputs:

```rust
#[derive()]
pub enum Test {
    One,
    Two,
}
```
and a :( from me.

Thanks guys

