debian-control 0.2.4

# Bug: ensure_minimum_version() loses newline-after-colon formatting

## Description

When using `Relations::ensure_minimum_version()` to update a dependency version, the formatting is not preserved. Specifically, when the original has a newline after the field name colon, the serialized output loses this newline and puts the first entry on the same line as the field name.

## Expected Behavior

The formatting should be preserved. If the input has:
```
Build-Depends:
 debhelper (>= 9),
 pkg-config,
 uuid-dev
```

After calling `ensure_minimum_version("debhelper", "12~")`, it should produce:
```
Build-Depends:
 debhelper (>= 12~),
 pkg-config,
 uuid-dev
```

## Actual Behavior

Instead it produces:
```
Build-Depends: debhelper (>= 12~),
 pkg-config,
 uuid-dev
```

The newline after the colon is lost.

## Reproduction

```rust
use debian_control::lossless::Control;
use debversion::Version;
use std::str::FromStr;

let input = r#"Source: f2fs-tools
Section: admin
Priority: optional
Maintainer: Test <test@example.com>
Build-Depends:
 debhelper (>= 9),
 pkg-config,
 uuid-dev

Package: f2fs-tools
Description: test
"#;

let control = Control::from_str(input).unwrap();
let mut source = control.source().unwrap();
let mut build_depends = source.build_depends().unwrap();

let version = Version::from_str("12~").unwrap();
build_depends.ensure_minimum_version("debhelper", &version);

source.set_build_depends(&build_depends);

println!("{}", control.to_string());
// The Build-Depends line will have lost its newline after the colon
```

## Environment

- debian-control: 0.2.3
- deb822-lossless: 0.4.3

## Impact

This affects lintian-brush fixers that need to update dependency versions while preserving the original formatting style of control files.