Change request HTTP headers to comply with SOAP 1.1/1.2 spec

In SOAP 1.1, the SOAPAction HTTP request header is mandatory even if its value is a quoted empty string (indicating that  the intent of the SOAP message is provided by the HTTP Request-URI) or not specified at all (which means that there is no indication of the intent of the message).
In SOAP 1.2 (according to [RFC3902](http://www.w3.org/TR/soap12-part2/#soap-media-type)), this header  was replaced with the optional *action* attribute of the *application/soap+xml* media type.