[DIFE-196] Use a correct Ruby syntax to initalize SOAP client

We were using an old syntax to initialize the SOAP client that, in some conditions, might cause problems. We got that syntax from the EC example code. Initially we didn't have that problem, but after !81 (merged) the problem appeared. It's caused by a misuse of a Ruby syntax.

This MR uses a better syntax to initialize the client. This syntax is the official one (check https://github.com/savonrb/savon#usage-example) and it avoids the problem we were having.