demo

internal: non-strict RFC etag support

For some caldav providers, it wasn't possible to unmarshal unquoted text for etag.
https://datatracker.ietf.org/doc/html/rfc2616#section-3.11 points to wrap text with quotes, but in reality it's sometimes ignored.

internal: test for ETag UnmarshalText, ETag String, ETag unmarshal and marshal