bdk: Call map_keychain in unbounded_spk_iter

Every other Wallet method uses map_keychain to make sure to return results
for KeychainKind::External if the method is called with
KeychainKind::Internal and the internal descriptor doesn't exist.

crates/bdk/src/wallet/mod.rs

@@ -932,6 +932,7 @@ impl<D> Wallet<D> {
         &self,
         keychain: KeychainKind,
     ) -> impl Iterator<Item = (u32, ScriptBuf)> + Clone {
+        let keychain = self.map_keychain(keychain);
         self.indexed_graph.index.unbounded_spk_iter(&keychain)
     }