Fixed MPCLOVER-38, maven:set doesn't allow null values

git-svn-id: https://svn.apache.org/repos/asf/maven/maven-1/plugins/trunk@178218 13f79535-47bb-0310-9956-ffa450edef68

clover/plugin.jelly

@@ -222,8 +222,8 @@
     <j:set var="maven.build.dest" value="${mavenBuildDestOld}" scope="parent" />
     ${context.removeVariable('build.compiler')}
 
-    <maven:set plugin="maven-test-plugin" property="maven.test.failure.ignore"
-        value="${ignoreTestFailureOld}"/>
+    ${pom.getPluginContext('maven-test-plugin').setVariable(
+        "maven.test.failure.ignore", ignoreTestFailureOld)}
     <maven:set plugin="maven-test-plugin" property="maven.junit.fork"
         value="${junitForkOld}"/>