No single report generated with latest maven-reporting-impl

Recently, I fixed several report plugins (changelog, changes etc.) to use Doxia 1.0 and latest maven-reporting-impl.
The main goal was to make reporting plugins available in PDF (see MPDF-26 and doc [1])

I notified that running a single report doesn't work but works when coupling with maven-site-plugin 2.0.x.

For instance, run:

mvn org.apache.maven.plugins:maven-changelog-plugin:2.2-SNAPSHOT:changelog

In Doxia 1.0, the SiteRenderSink [2] uses a StringWriter by default to use getBody() in the renderer [3]. So running a single report doesn't write a file with reporting-impl:2.0.4.2

In MPIR 2.1.2, we overrided the mojo.execute() to handle a default renderer [4] so we are able to run a single report. I propose to move this logic in AbstractMavenReport.

[1] http://maven.apache.org/plugins/maven-pdf-plugin-1.1-SNAPSHOT/examples/configuring-reports.html#Maven_Reporting_Plugins_Issues
[2] http://maven.apache.org/doxia/doxia-sitetools-1.0.x/xref/org/apache/maven/doxia/siterenderer/sink/SiteRendererSink.html#47
[3] http://maven.apache.org/doxia/doxia-sitetools-1.0.x/xref/org/apache/maven/doxia/siterenderer/DefaultSiteRenderer.html#433
[4] http://maven.apache.org/plugins/maven-project-info-reports-plugin/xref/org/apache/maven/report/projectinfo/AbstractProjectInfoReport.html#137