Filtering doesn't support default for rev flagging

[jelovirt]

For DITAVAL

<val>
  <revprop action="flag" color="red"/>
</val>

no flagging markup is generated. The spec states

If the val attribute is absent, then the revprop element declares a default behavior for any value in the rev attribute.

Same applies to prop element, e.g.

<val>
   <prop action="flag" color="red" />
</val>

[jelovirt]

@robander, do you think you could get a fix for this in 1.7 still?