Adds check for date header and value

[MollieS]

• Ensures a response for a partial get request has a date header and date

[damonkelley]

I think we can add this, but we might want to tweak it first. The spec indicates that this field is only required when the Date header is also included in the response when the entire resource is requested. We are not checking for the Date header for any other resources, so it feels a little weird to only check for it in a partial content response.

When a 206 response is generated, the server MUST generate the
following header fields, in addition to those required above, if the
field would have been sent in a 200 (OK) response to the same
request: Date, Cache-Control, ETag, Expires, Content-Location, and
Vary.

https://tools.ietf.org/html/rfc7233#section-4.1

[damonkelley]

I am trying to get take care of this backlog of PRs that we have accumulated. I acknowledge that this PR was opened ~1.5 years ago, so if you think you are not going to be able to move this PR forward, let me know.