jacoco-maven-plugin and maven-surefire-plugin conflict on ${argLine}

This problem is reproducible with maven-surefire-plugin version 2.17 and jacoco-maven-plugin versions 0.7.1.201405082137 and 0.7.2.201409121644. However it functions perfectly OK with 0.6.4.201312101107.

Configure surefire like
```
                <plugin>
                    <groupId>org.apache.maven.plugins</groupId>
                    <artifactId>maven-surefire-plugin</artifactId>
                    <configuration>
                        <argLine>${argLine} -Xmx1024m -XX:MaxPermSize=256m</argLine>
                    </configuration>
                </plugin>
```
and it will result in 
```
[INFO] --- jacoco-maven-plugin:0.7.1.201405082137:prepare-agent (jacoco-initialize) @ automated-deployment-system ---
[INFO] argLine set to -javaagent:/work/m2_repo/org/jacoco/org.jacoco.agent/0.7.1.201405082137/org.jacoco.agent-0.7.1.201405082137-runtime.jar=destfile=/work/jenkins_data/jobs/nightly-build-automated-deployment-system/workspace/target/jacoco.exec -ea
[INFO] 
[INFO] --- jacoco-maven-plugin:0.7.1.201405082137:report (jacoco-site) @ automated-deployment-system ---
[INFO] Skipping JaCoCo execution due to missing execution data file:/work/jenkins_data/jobs/nightly-build-automated-deployment-system/workspace/target/jacoco.exec
.....
[INFO] [10:35:38.754] Project coverage is set to 0% as no JaCoCo execution data has been dumped: /work/jenkins_data/jobs/nightly-build-automated-deployment-system/workspace/rpm/target/jacoco.exec
```
in logs