Sets an ETag for the fragment's underlying data.
ETags can be used to skip unnecessary rendering of unchanged content.
See Conditional requests for a full example.
How [up-etag] attributes are set
Unpoly will automatically set an [up-etag] attribute when a fragment was rendered
from a response with a ETag header. When a fragment was rendered without such a header,
Unpoly will set [up-etag=false] to indicate that its ETag is unknown.
A large response may contain multiple fragments that are later reloaded individually
and should each have their own ETag. In this case the server may also also render multiple
fragments with each their own [up-etag] attribute.
See Individual versions per fragment for an example.
