Fix the Jacoco Coverage Trend portlet

[dcendents]

Since the last update (remove joda-time dependency), the jacoco coverage trend chart portlet is broken. The java.util.Calendar toString method does not return a valid date string and the chart is all messed up:

This fix will format the Calendar into a String ("yyyy-MM-dd") and pass that value to JFreeChart:

Regards

[dcendents]

I've changed the commit, turns out to be correct the date needs to be converted into a string earlier to group results correctly in the summaries map.

Here's the correct result:

[centic9]

Good catch! Thanks for the PR!