The ability to add mutliple scopes for a Call

[elpiel]

I've made this small Trait extension for adding multiple scopes at once to a specific *Call.
It really helps when you have a static defining your scopes per request.

Can this be added to the *Call structs for convenience in some way?

use gmail1::api::{Scope, UserMessageGetCall, UserMessageListCall};

/// Extension trait for all Gmail calls.
pub trait CallExt {
    fn add_scopes(self, scopes: &[Scope]) -> Self;
}

impl<'a> CallExt for UserMessageGetCall<'a> {
    fn add_scopes(mut self, scopes: &[Scope]) -> UserMessageGetCall<'a> {
        for scope in scopes {
            self = self.add_scope(scope);
        }

        self
    }
}
impl<'a> CallExt for UserMessageListCall<'a> {
    fn add_scopes(mut self, scopes: &[Scope]) -> UserMessageListCall<'a> {
        for scope in scopes {
            self = self.add_scope(scope);
        }

        self
    }
}

[Byron]

This does indeed look useful to have, and a PR would be very welcome. If you consider submitting one, please don't regenerate the APIs as it makes the review difficult. Thanks.