Everything appears to be working correctly

src/main/resources/execute_asciidoctor.rb

@@ -1,11 +1,11 @@
 require 'asciidoctor'
 
-asciidoctor_opts = {:safe => Asciidoctor::SafeMode::UNSAFE, :base_dir => $srcDir}
+asciidoctor_opts = {:safe => Asciidoctor::SafeMode::UNSAFE, :base_dir => $srcDir, :backend => $backend}
 Dir.new($srcDir).each do |file|
   file_ext = File.extname(file.to_s)
   if file_ext === '.adoc' or file_ext === '.asciidoc' or file_ext === '.asc' or file_ext === '.ad'
     basename = File.basename(file.to_s, file_ext) + ($backend === 'docbook' ? '.xml' : '.html')
-    doc = Asciidoctor::Document.new(file.lines.to_a, asciidoctor_opts)
-    File.new($outputDir + '/' + basename, 'w+').puts doc.render
+    rendered_output = Asciidoctor::Document.new(file.lines.to_a, asciidoctor_opts).render
+    File.open($outputDir + '/' + basename, 'w') { |f| f.write(rendered_output) }
   end
 end

src/test/groovy/org/asciidoc/maven/test/AsciidoctorMojoTest.groovy

@@ -42,6 +42,6 @@ class AsciidoctorMojoTest extends Specification {
             outputDir.list().toList().contains('sample.xml')
 
             File sampleOutput = new File('sample.xml', outputDir)
-            sampleOutput.totalSpace > 0
+            sampleOutput.length() > 0
     }
 }