Allow property expansion for mux keys.

[alcarraz]

Property expansion is made in toStringArray to avoid repetitive code. But it could be explicit in the method call if preferred.

Also, set default key only once. This could be undone, but it was getting a child of the parent in the loop instead of using the content of the child, so the effect is the same.

Basically, I want to be able to do this:

<mux name="probe-mux" class="org.jpos.q2.iso.QMUX"
     enabled="${probe.enabled:true}"
     logger="${probe.logger:Q2}" realm="probe-mux">
  <key>${probe.mux.key:11,41}</key>
  <in>probe-receive</in>
  <out>probe-send</out>

  <ready>probe-channel.ready</ready>
</mux>