Allow to pass the `adapter` option in initializer, pass it to HTTPI when fetching remote WSDL

It will be useful for those people who need to use different HTTPI adapters for different services, see savonrb/savon#564

I've bumped gem version as there is related pull request in savon that depend on it: savonrb/savon#566.

I don't know, how to test this, please tell me any ideas and suggestions, I'll write tests and append it to this request.