mainClass needs to be optional for library() case

Author: mkristian

jruby-gradle-jar-plugin/src/main/groovy/com/github/jrubygradle/jar/JRubyJar.groovy

@@ -100,6 +100,7 @@ class JRubyJar extends Jar {
      * @return configured 'Main-Class' attribute for the JRubyJar
      */
     @Input
+    @Optional
     String getMainClass() {
         return jarMainClass
     }

jruby-gradle-jar-plugin/src/test/groovy/com/github/jrubygradle/jar/JRubyJarSpec.groovy

@@ -103,4 +103,19 @@ class JRubyJarSpec extends Specification {
         then:
         task.configuration == customConfig.name
     }
+
+    def "configure library()"() {
+        given: 'a task with initScript library()'
+        boolean evaluated = false
+        JRubyJar task = project.task('library-jar', type: JRubyJar)
+
+        when:
+        task.configure {
+            evaluated = true
+            initScript library()
+        }
+
+        then:
+        evaluated
+    }
 }