no suggestion for possible value when using 'long=' format

[kbknapp]

When setting up a long option with possible values, and you run --long=value if value isn't valid you will not get an error. Yet running --long value will result in the appropriate error.

Example test:

static VALUES: [&'static str; 2] = ["possible", "vals"];

fn main() {
    App::new("test").arg(Arg::from_usage("--option <val> 'some option'").possible_values(&VALUES)).get_matches();
}