Change base64 xelement logic and add test to verify encoding / decoding behavior
It appears that the XName expected by XElement.Element(XName)
was Base64
in the case of e.g. testplans, but {http://opentap.io/schemas/package}Base64
in the case of package xml.
I have updated the seralizer logic to consider only the LocalName of XElements so it will work in both instances.
Closes #722 (closed)