Does Xamarin not support ASMX service with SOAP 1.2?

I have problems with using an ASMX web service that supports SOAP version 1.2.

Specifically, the action parameter is not added to Content-Type of HTTP header in my requests.

The docs state:

The Xamarin platform supports standard SOAP 1.1 implementations over HTTP

I tried to write my own SoapHttpClientProtocol derived class, and override the ContentType, but it didn't work.

I have to write service classes myself, which will generate much work. When all that is missing is a simple action parameter.