bugfix: supporting namespace for querying the same attribute name with different namespace

[codelynx]

bugfix: when a node has two attributes "id" and "r:id", node["r:id"] will not find its attribute value, by checking ns (namespace) to be able to find right attribute obscured by the same attribute name with different namespace.

Following example demonstrates the attribute "r:id" will be found by node["r:id"]. Because, subscript(attributeName: String) only checks tagName, then "r:id" will be obscured by "id".

<p:sldIdLst>
    <p:sldId id="256" r:id="rId2" />
    <p:sldId id="257" r:id="rId3" />
</p:sldIdLst>