Header types validation support

[DXist]

At the moment I can't enforce lxml validation of header simple or complex types.

[plq]

you're not alone :P

For future reference, here's where soap body validation is done. probably header validation should be done here as well. https://github.com/arskom/spyne/blob/591b72d59980d81678d2979ea0a76bd8a81614ec/spyne/protocol/soap/soap11.py#L210

I can't remember why we have to validate in decompose call though. Isn't it better to do it right before deserialization?

[DXist]

I think envelope decomposition and validation are different tasks that can be done independently.