The conditions of an HTTP conditional request.
Checks validity based on the given ETag and/or date.
This function assumes the resource being checked exists.
If not, the
matchesMetadata function should be used.
Condition based on ETag.
Condition based on last modified date.
Valid if matching any of the given ETags.
Valid if modified since the given date.
Valid if not matching any of the given ETags.
Valid if not modified since the given date.