GraphML serialization invalid if a vertex and edge have similar named property

I have created a graph using the Tinkerpop Graph and required for Gephi visualization a property called color on edges and vertices to be coloured.

The current gremlin-core serialization creates the following two key elements in GraphML

<key id="color" for="node" attr.name="color" attr.type="string"></key>
<key id="color" for="edge" attr.name="color" attr.type="string"></key>

the id attribute is an internal name, but have to be different.

I would suggest a patch to check for an intersection of the edge & vertices keys and add for those keys an additional differentiating letter like:

<key id="colorV" for="node" attr.name="color" attr.type="string"></key>
<key id="colorE" for="edge" attr.name="color" attr.type="string"></key>

Going to provide a pullrequest on GitHub.