Missing `etag` for collections

[WhyNotHugo]

Baikal version: 0.8.0

Expected behaviour:

An etag is missing for collections

Current behaviour:

etag is missing from response.

Steps to reproduce:

Send the following request to the calendar home set:

<propfind xmlns="DAV:"><prop><resourcetype/><getetag/></prop></propfind>

Notes:

getetag support is mandatory, and is required in order to prevent in-flight conflicts (e.g.: to prevent deleting a collection while another client has modified it).