NEXUS-5747: Support packaging bundle

Added bundle to defaults and added UT for
DefaultArtifactPackagingMapper.

Note: while issue points at ASF Maven Indexer
mapper, the Nexus Maven Indexer integration actually
redirects MI to this (changed) component. So,
this applies the fix to Nexus, but does not
applies the same to MI...
Merge pull request #32 from sonatype/nexus-5747-bundle-packaging

NEXUS-5747: Support packaging bundle