Support in SOAPClient for using XMLs built with 'mkp.yield'

[agseco]

This allows to modularise XMLs generation, for instance breaking down each XML part into a different method and then putting all the parts together with mkp.yield.

For illustration purposes:

def response = client.send {
    body {
        CoreQuery {
            mkp.yield this.buildCoreQueryElement(ctx)
        }
    }
}

def buildCoreQueryElement(ctx) {
    return {
        ElementA('Some content')
    }
}

Currently this approach won't work because the XML serialisation to String performed in SOAPMessageBuilder is carried out using groovy.xml.MarkupBuilder; groovy.xml.StreamingMarkupBuilder could be used instead, as in RESTClient.