Custom reference.uri is not honored by SignedXml.prototype.createReferences

[ergoithz]

Currently there is no way to customize the reference URI, even if SignedXml.prototype.addReference accepts it as argument, because is always overwritten by xpath id attribute.

https://github.com/yaronn/xml-crypto/blob/6e22854b1aeb4e59bd482a185826361a39872704/lib/signed-xml.js#L584

https://github.com/yaronn/xml-crypto/blob/6e22854b1aeb4e59bd482a185826361a39872704/lib/signed-xml.js#L443

[cjbarth]

@ergoithz , we'd welcome a PR with a test suite to address this limitation in xml-crypto. Would you consider contributing to the community to address this weakness?