new-without-default-derive ignores existing derives

[llogiq]

To make applying the Default derive easier, we could look at existing derives and add Default there.

[wduquette]

Still seeing this error sometimes; here's an example. This is with Rust 2018 Edition and "stable-x86_64-apple-darwin unchanged - rustc 1.31.1 (b6c32da9b 2018-12-18".

#[derive(Default)]
pub struct Foo<T> {
    bar: Option<Bar<T>>,
}

impl<T> Foo<T> {
    pub fn new() -> Self {
        Self {
            bar: None
        }
    }
}

struct Bar<T> {
    quux: T,
}