Support java_args being specified by both a string and an array.

This patch restores support for arrays that was lost after change 4e24616239d8c455f51e1b2058c3e83060731511
diff --git a/addon/buildr/jacoco.rb b/addon/buildr/jacoco.rb
index fbb0b0a..8bddf81 100644
--- a/addon/buildr/jacoco.rb
+++ b/addon/buildr/jacoco.rb
@@ -138,7 +138,8 @@
             options << "excludes=#{project.jacoco.excludes.join(':')}" unless project.jacoco.excludes.empty?
 
             agent_config = "-javaagent:#{agent_jar}=#{options.join(',')}"
-            project.test.options[:java_args] = ([project.test.options[:java_args]] || []) + [agent_config]
+            existing = project.test.options[:java_args] || []
+            project.test.options[:java_args] = (existing.is_a?(Array)? existing : [existing]) + [agent_config]
           end
           namespace 'jacoco' do
             if project.jacoco.generate_xml?