Add PR to pending change log
Adds PR to pending change log.
### SDK Enhancements
### SDK Bugs
+* `private/model/api`: Fix SDK's unmarshaling of unmodeled response payload ([#2340](https://github.com/aws/aws-sdk-go/pull/2340))
+ * Fixes the SDK's unmarshaling of API operation response payloads for operations that are unmodeled. Prevents the SDK due to unexpected response payloads causing errors in the API protocol unmarshaler.
+ * Fixes [#2332](https://github.com/aws/aws-sdk-go/issues/2332)
