Edges weight not merged with parallel edges

[Mathieu Bastian]

Way to reproduce
1) Import Graph file with two parallel edges

<?xml version="1.0" encoding="UTF-8"?>
<gexf xmlns="http://www.gexf.net/1.1draft" version="1.1">
    <graph mode="static" defaultedgetype="directed">
        <nodes>
            <node id="0" label="Hello" />
            <node id="1" label="Word" />
        </nodes>
        <edges>
            <edge source="0" target="1" />
            <edge source="0" target="1" />
        </edges>
    </graph>
</gexf>

The edge weight should be merged, and have value equal to 2.0.