Error "No karaf base dir found in extracted distribution." when using customized Karaf distribution

Following the manual at http://karaf.apache.org/manual/latest-2.3.x/developers-guide/writing-tests.html#KarafDistributionConfigurationOption I tried to write integration tests based on a custom Karaf distribution.
I used the following to configure the container:

...
karafDistributionConfiguration()
	.frameworkUrl(
			maven()
			.groupId("at.ac.ait.hbs.homer")
			.artifactId("at.ac.ait.hbs.homer.core.distribution.assemblies.stable")
			.type("zip")
			.versionAsInProject()
	)
	.karafVersion(KARAF_VERSION)
	.name("Apache Karaf")
	.unpackDirectory(new File(UNPACK_DIRECTORY)),
...

When running the tests I get an "No karaf base dir found in extracted distribution." error. When looking into the "target\exam\<random>\" folder (= UNPACK_DIRECTORY) there is really a mess:

DISCLAIMER.txt
LICENSE.txt
README.txt
...
bin\
...
config.properties
custom.properties
endorsed\
ext\
...
karaf.bat
karaf.jar
keys.properties
org\
...
system.properties
tmp\
users.properties

What I expected was the default Karaf distribution layout:

bin\
data\
demo\
deploy\
etc\
lib\
system\
README
karaf-manual-2.3.2.html
karaf-manual-2.3.2.pdf
...

Digging into the problem I found out that the eg. the apache-karaf-2.3.2.zip looks like:

apache-karaf-2.3.2\bin\
apache-karaf-2.3.2\data\
apache-karaf-2.3.2\demo\
apache-karaf-2.3.2\deploy\
apache-karaf-2.3.2\etc\
apache-karaf-2.3.2\lib\
apache-karaf-2.3.2\system\
apache-karaf-2.3.2\README
apache-karaf-2.3.2\karaf-manual-2.3.2.html
apache-karaf-2.3.2\karaf-manual-2.3.2.pdf
...

Whereas the custom distribution I'm using looks like:

bin\
data\
demo\
deploy\
etc\
lib\
system\
README
karaf-manual-2.3.2.html
karaf-manual-2.3.2.pdf
...

Note the different "base" directory in the two packages - "apache-karaf-2.3.2" in case of the Karaf 2.3.2 assembly.
It seems that the container assumes that every zip/tar.gz file given has this "base" directory structure and extracts only the contents of subfolders to the unpack directory.

I think a check for this "base" directory should be integrated in the container and either extract only the contents in this "base" directory or extract the whole file otherwise - the base directory seems to have the same name as the artifact name?!

If you want to try it out you can get the distribution from http://dev.arcsmed.at/nexus/content/repositories/homer.core-snapshots/at/ac/ait/hbs/homer/at.ac.ait.hbs.homer.core.distribution.assemblies.stable/1.2.0-SNAPSHOT/