Fix position of auto in auto trait declaration

[tlively]

rustfmt 0.7.0-nightly (b0eb8993 2018-05-18) changes the following code

#![feature(optin_builtin_traits)]

pub auto trait Auto {}

which compiles cleanly, to

#![feature(optin_builtin_traits)]

auto pub trait Auto {}

which does not compile. This PR fixes the issue and adds tests for it.

[nrc]

Thank you!