Omit vendor extensions when creating types

[whitlockjc]

Whenever we process the paths or some other OAS type, we do not handle vendor extensions appropriately. This could lead to some x-* property beneath paths being turned into a Path object, and in some cases would not work due to a value being null or undefined. We need to ensure that we are omitting vendor extensions when processing the parsed types in the OAS document.

Originally reported here: apigee-127/swagger-tools#597