Allow diameter message & avp to normalize their xml representation

The method `asXMLString` for Message and Avp classes has now an
optional bool "sortAttributes" which defaults to `false` (previous
behaviour).

So, you could normalize by mean sorting attribute names for nodes
within xml. This does not affect to the structure because the
method asXMLString just generates the representations on-demand.

The new parameter is set to true when programming regexps and during
TestCondition comply for diameter messages received.