Add types to first level nodes

The types of first level nodes are present in the WSDL's message section for the request. Those can be added from there. I am not sure why wasabi document does not have an attribute accessor for @messages. I could have avoided the call to instance_variable_get. If you agree I can make that change in another pull request to wasabi. Adding of types to inner nodes will need me to check in @types. Before I try that I wanted to understand the reason of keeping arrays as keys for @types rather than having nested hash structure. Suggestions are welcome.