Workaround bogus http 401 in login method call

Some RETS servers return success code in XML body but failure code 4xx in http status. We already have a workaround for it in `Client::ErrorChecker#check`

https://github.com/estately/rets/blob/master/lib/rets/client.rb#L356

But we forgot to use it in `Client#login` method. This PR rectifies it.
the method is 'get' not 'post'. It was copy/paste error