Shading of multi-release jars produces an invalid manifest

[jeremyk-91]

See internal atlasdb proxy client, issue 1333: builds there are failing on being unable to read the manifest.

The integration spec also looks like it produces an invalid manifest: if I add these lines to the bottom of the test

        // What is of interest here is that this does not throw an exception
        shadowJarFile().getManifest().getAttributes("Multi-Release")

an exception on reading the manifest is thrown.

I believe the problem here is that ManifestResourceAppender simply adds key-value pairs to the end of a file. So you end up with a problem where if a manifest already ends with an empty line (which looks like something internal to the shadow plugin), then appending ends up getting

Manifest-Version: 1.0

Multi-Release: true

which is not a valid format; it should really be

Manifest-Version: 1.0
Multi-Release: true

I'd write a PR, but have struggled with dependencies/possible classpath issues for >2 hours...
Turns out 2.5 hours was the magic mark!