js_name does not seem to effect struct values

[hacker112]

Describe the Bug

js_name does not affect public values in structs like it affects methods.

Steps to Reproduce

Create a struct

#[wasm_bindgen]
pub struct FooBar {
    #[wasm_bindgen(js_name = "spamEggs")]
    pub spam_eggs: u32,
}

Expected Behavior

Should generate JS/TS with name spamEggs just like for methods.

OR throw an error that this is not possible.

Actual Behavior

Generates JS/TS with name spam_eggs as if the js_name attribute would not have been set.

[alexcrichton]

Should be easy enough to implement, but agreed that this isn't working today!