Not handling the encoding of explicit tags when a default value is present

[jusbeyer]

Recently rasn added support for explicit tags in an ASN.1 Structure. When this support was added, though, it does not appear that anything was added to handle the non-encoding default values.

Per ITU standard, this value should not be present during encoding. Since decoding automatically sets the default value, when not present, and will preserve the value, if provided, a fix should be implemented and will be backwards compatible (assuming downstream systems correctly handle this).

Example code:

use rasn::prelude::*;

#[derive(AsnType, Decode, Debug, Encode, PartialEq)]
pub struct SequenceWithMultiDefault {
    #[rasn(tag(explicit(application, 1)), default="default_bool")]
    b: bool,
    #[rasn(tag(explicit(application, 2)), default="default_bool")]
    b2: bool,
}
pub fn default_bool() -> bool {
    true
}

fn main() {
    let seq = SequenceWithMultiDefault{ b: true, b2: false };
    let encoded = rasn::der::encode(&seq).unwrap();
    println!("{:02x?}", encoded);
}

Produces:

[30, 0a, 61, 03, 01, 01, ff, 62, 03, 01, 01, 00]

Should Produce (Note: The first tagged field is not present):

[30, 05, 62, 03, 01, 01, 00]