Fix for https://github.com/awslabs/aws-sdk-java-resources/issues/14

The way we currently handle single-valued request identifiers assumes there's at least one response identifier to tell us how many copies to make. For cases like IAM's CreateUser (where the only identifier is the name of the user mapped in from the request), this assumption is invalid.

This change moves handling of single-valued request identifiers up alongside parent identifiers, which are handled correctly.