Ruby: Ensuring that HTTP failures will clear the http transport outbuf var

PR: https://github.com/apache/thrift/pull/1048

With the current implementation, any Net HTTP failure will raise from the #flush() method without resetting the @outbuf variable.

I think that resetting the @outbuf on these failures is more "expected" behaviour. Especially if there is a malformed request that the downstream server does not want to/can't handle. As far as I can tell, there is not way to clear the @outbuf var apart from the #flush() method, so if that fails, then you will just keep appending requests to the out buffer.