fix when response is empty

[jrunge]

i was getting an error when the response from amazon came back empty. in that case, the parser returns result as null rather than undefined. checking for result.Errors then crashes node.

my fix was to check if data === '' before running the parser.

[miccolis]

I suspect the root of this problem isn't that we get an empty response, but that we're not checking the http response code from AWS at all. There is another pull ( #48 ) that also has a partial fix here.

It seems like we should be able immediately error out on a status code > 500, and for others we should figure out a way attempt a parse when there is data, but error our with the status code if there isn't any or if it's not xml.

Does anyone know exactly when AWS would return a useful error in XML or when we could just use the http code?