Use var for attrPrefix and contentPrefix

[henripqt]

As of now we cannot remove attrPrefix and contentPrefix by setting them to a blank string as the decoder checks on that and defaults to their const attrPrefix and contentPrefix

In our case we'd like not to use any prefixes onto either of the extracted values. A simple fix could be to define attrPrefix and contentPrefix as var instead of const in order to allow overriding them using ldflags

[M4x3e3e]

same issue here

[NikolayUvarov]

Cannot rid of "minuses" before inner parameter names in a simple way. Conversion to parameters that has different name (with minus prefix) is an obstacle.

update:
The issue mostly due to lack of dockumentation.
Here as I have solved the prefix problem:

json, err := xj.Convert(xml, xj.WithAttrPrefix(""))

It seems reasonable than by default there were no prefixes and user should add them conciously.