Take subgroups into account when using a group clause in an action filter rule

When using the <group> clause in an action filter rule, subgroups are not taken into account.
For example, lets say you have:

<filter id="myFilter">
  <rule grant="true">
    <group>testgroup</group>
  </rule>
</filter>

if a user belongs to a subgroup of the testgroup group, the filter will return false.
It would be nice to handle subgroups as well.
The fix looks trivial: using principal.isMemberOf(...) instead of principal.getGroups().contains(group) in DefaultActionFilter#checkGroups.