arguments and commandlineArgs seem to be mutually exclusive

When I specify both arguments and commandlineArgs when executing the java goal, the latter supplants the former value.

                <plugin>
                    <groupId>org.codehaus.mojo</groupId>
                    <artifactId>exec-maven-plugin</artifactId>
                    <version>1.6.0</version>
                    <configuration>
                        <mainClass>${mainClass}</mainClass>
                        <commandlineArgs>-Duser.dir=${project.basedir}</commandlineArgs>
                        <arguments>
                            <argument>server</argument>
                        </arguments>
                    </configuration>
                </plugin>

In this case, I would like to pass arguments to the JVM and my class that differ. In fact the log seems to indicate that is actually what happens:

[DEBUG] Invoking : com.whoever.Application.main(server)

However, in reality the arguments configuration is overwritten by the commandlineArgs within the private method ExecJavaMojo.addRelevantPluginDependenciesToClasspath. This seems to be incorrect behavior... at least when arguments configuration is supplied.

Start by reading the java goal handling around the private method ExecJavaMojo.addRelevantPluginDependenciesToClasspath and reproduce the configuration shown in the issue. Trace how arguments and commandlineArgs are combined when both are supplied. Done means the JVM receives the command-line option while Application.main still receives server, with coverage for this configuration if the existing tests provide a suitable entry point.