Fix xml to json converter

When an array has a unique element, xmltodict does not
parse it as an array, but a single object. This causes
a problem in the diameter xml conversion because the
library for reverse conversion [0] won't follow the
diameter message dtd template.

Fortunately, xmltodict supports [1] force array for specific
node name, so we will force this for 'avp' which is the
one affected.

[0] https://github.com/testillano/json2xml
[1] https://stackoverflow.com/questions/37207353/xmltodict-does-not-return-a-list-for-one-element