Attempt to fix aws/ec2 with :filters parameter

[Gleefre]

Probably fixes https://github.com/pokepay/aws-sdk-lisp/issues/30

Disclaimer: I'm not sure at all that it fixes the issue - I don't have enough knowledge to be sure that it doesn't break anything, and I don't even know how to test if this works properly - I don't use aws.

Still, I hope that it is a correct fix or it will be at least useful.