Regression: date of last changelog entry is lost

[rmunn]

If a changelog has only one entry in it (say, because it's just been created from a template) then Changelog.LatestEntry will contain Date = None even if there was a date in the changelog. This is likely to cause build scripts to fail; for example, the default build script from https://github.com/TheAngryByrd/MiniScaffold contains the following logic in the "DotnetPack" target:

let releaseDate = latestEntry.Date.Value.ToString("o")

I believe this regression was introduced by #2779, specifically by these two lines. They call the ChangelogEntry.New overload that takes three parameters, which ends up removing the date from the already-parsed entry. They should be replaced by:

let newLastEntry =
    ChangelogEntry.New(last.AssemblyVersion, last.NuGetVersion, last.Date, last.Description, trueChanges, last.IsYanked)

To reproduce

Put the following into CHANGELOG.md and have Fake.Core.ReleaseNotes parse it:

# 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/),
and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).

## [Unreleased]

## [0.1.0] - 2017-03-17
First release

### Added
- This release already has lots of features

[0.1.0]: https://github.com/fsprojects/FAKEreleases/tag/v0.1.0

Then print out Changelog.LatestEntry and you'll see Date = None instead of Date = Some 2017-03-17.