Add Collections to X- Header list

[varnerac]

We provide two headers useful for pagination outside of the standard way:

• X-TAXII-Date-Added-First
• X-TAXII-Date-Added-Last

They are required for:

• GET <api-root>/collections/objects/
• GET <api-root>/collections/manifest/

They should also be required for GET /<api-root>/collections/, since it also requires pagination support.

[jordan2175]

So the first and last dates would just be the same for the case of a collection though right?

[varnerac]

After the Slack discussions on pagination, I'm not convinced this is a good idea. Instead, I think pagination of Collections should be my the Collection id as @MarkDavidson suggested. Time-based pagination alone is inadequate.

[varnerac]

OBE. No point in fixing his given added_after pagination in 2.1