MULE-7189: Support RPC Stype with proxy-service and proxy-client

https://www.mulesoft.org/jira/browse/MULE-7189

When proxying a web service call with RPC style using proxy service and proxy client, the content of the body is being truncated and the invocation fails because it can't forward an invalid content to the final service. The solution is to avoid using CXF's RPCInInterceptor and making sure that the correct operation is set with a new interceptor.