Fix describe_internet_gateways calls for >1 igw

@internet_gateway was incorrectly reset after the first
InternetGatewaySet item.  On encountering a second, we end up
attempting to dereference nil as a hash, causing the parser to
blow up with an unintuitive error.
Merge pull request #3045 from jtopper/fix_describe_internet_gateways

Fix describe_internet_gateways calls for >1 igw