ARGO-279:  fix Dor::Workflow::Document#to_solr to index the right status ('completed') when all steps in a workflow are 'completed' or 'skipped'.  update unit tests.
improve formatting of xml in unit tests
Merge pull request #21 from sul-dlss/workflow_status_to_solr_ARGO-279

Workflow status to_solr
Version bump to 4.15.0