Add-ChangelogData wipes Unreleased data

[codaamok]

Current behavior

Say my CHANGELOG has the below contents. If I issue Add-ChangelogData -Type Added -Data "Added something new", then all of the Unreleaed content gets replaced with the previously passed data.

This is using 2.1.4. At the time of writing this, that's latest available from the PowerShell gallery.

# Changelog
All notable changes to this project will be documented in this file.

The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).

## [Unreleased]
### Added
- Added Get-DP function
- Added Get-DPGroup function

### Changed
- Get-DPDistributionPointStatus now accepts value from pipeline or an array of distribution points

### Fixed
- Unable to pipe result of some functions to other functions due to ObjectType property often being an enum rather than string

## [1.3.20200821.4] - 2020-08-21
### Added
- Bundled license file and change log with payload
- Implemented build process to streamline deployment
- Added DeploymentId to Find-CMObject

## [1.2] - 2020-07-30
### Added
- Birth of change log

[Unreleased]: https://github.com/codaamok/PSCMContentMgmt/compare/1.3.20200821.4..HEAD
[1.3.20200821.4]: https://github.com/codaamok/PSCMContentMgmt/compare/1.2..1.3.20200821.4
[1.2]: https://github.com/codaamok/PSCMContentMgmt/tree/1.2

Result after running Add-ChangelogData -Type Added -Data "Added something new":

# Changelog
All notable changes to this project will be documented in this file.

The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).

## [Unreleased]
### Changed
- Added something new

## [1.3.20200821.4] - 2020-08-21
### Added
- Bundled license file and change log with payload
- Implemented build process to streamline deployment
- Added DeploymentId to Find-CMObject

## [1.2] - 2020-07-30
### Added
- Birth of change log

[Unreleased]: https://github.com/codaamok/PSCMContentMgmt/compare/1.3.20200821.4..HEAD
[1.3.20200821.4]: https://github.com/codaamok/PSCMContentMgmt/compare/1.2..1.3.20200821.4
[1.2]: https://github.com/codaamok/PSCMContentMgmt/tree/1.2

Expected behavior

I expected "Added something new" to get appended to the ### Added heading within the Unreleased section.

Steps to replicate behavior (include URLs)

1. Please see above

Screenshots

Not necessary (IMO)