Fix wif network

[mcroad]

Description

BDK generates WIFs in the wrong format when converting an ExtendedKey into an ExtendedPrivKey via into_xprv.
Specifically, all generated WIFs are in Network::Bitcoin format instead of the specified network.

let xkey: ExtendedKey = bip39::Mnemonic::parse_in(
    bip39::Language::English,
    "jelly crash boy whisper mouse ecology tuna soccer memory million news short",
)
.unwrap()
.into_extended_key()
.unwrap();
let xprv = xkey.into_xprv(Network::Testnet).unwrap();
let wif = PrivateKey::from_wif(&xprv.private_key.to_wif()).unwrap();

// assertion fails
assert_eq!(wif.network, Network::Testnet);

Notes to the reviewers

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[notmandatory]

ACK ee41d88

Nice catch.