Allow custom xml.Decoder to be used when creating a new XmlPullParser

[bluemods]

My use case stands to benefit from passing in an custom XML decoder, but since the decoder field is lowercase, I can't instantiate it outside of the package and I can't pass a custom decoder.

See here for an example: https://github.com/bluemods/kik-go-proxy/blob/e9db8eef9aaa59ce3c102a9402fa9181d1f17bfe/node.go#L57C8-L57C8

I want to use "k" as an argument to AutoClose to fix my issue, but I can't.

[bluemods]

I ended up fixing it, but it still may be something to add to this repo, if the maintainers are interested.