PROPPATCHing lastmodified property clears checksums

Steps to reproduce

1. PROPPATCH the lastmodified property of a resource, e.g. in the following way:

Request:
curl -u admin:admin "http://localhost:8080/remote.php/dav/files/admin/test.txt" -X PROPPATCH --data-binary @"patch.xml"

patch.xml:

<?xml version="1.0" encoding="utf-8" ?>
<a:propertyupdate xmlns:a="DAV:" xmlns:oc="http://owncloud.org/ns">
	<a:set>
		<a:prop>
			<a:lastmodified>1234</a:lastmodified>
		</a:prop>
	</a:set>
</a:propertyupdate>

Response:

<?xml version="1.0"?>
<d:multistatus xmlns:d="DAV:" xmlns:s="http://sabredav.org/ns" xmlns:oc="http://owncloud.org/ns">
	<d:response>
		<d:href>/remote.php/dav/files/admin/test.txt</d:href>
		<d:propstat>
			<d:prop>
				<d:lastmodified/>
			</d:prop>
			<d:status>HTTP/1.1 200 OK</d:status>
		</d:propstat>
	</d:response>
</d:multistatus>

Expected behaviour

The checksums of the file should not be changed.

Actual behaviour

The checksums of the file were cleared, as can be seen in the database:

+--------+---------+----------------+----------+------+-------+---------------+----------------------------------+----------+
| fileid | storage | path           | name     | size | mtime | storage_mtime | etag                             | checksum |
+--------+---------+----------------+----------+------+-------+---------------+----------------------------------+----------+
|  10246 |       1 | files/test.txt | test.txt |  253 |  1234 |          1234 | 7c065477017d493827737604c320d3c8 |          |
+--------+---------+----------------+----------+------+-------+---------------+----------------------------------+----------+

PROPFIND:

Request:
curl -u admin:admin "http://localhost:8080/remote.php/dav/files/admin/test.txt" -X PROPFIND --data-binary @"find.xml"

find.xml:

<?xml version="1.0" encoding="UTF-8"?>
<d:propfind xmlns:d="DAV:" xmlns:oc="http://owncloud.org/ns">
	<d:prop>
		<d:getlastmodified/>
		<oc:checksums/>
	</d:prop>
</d:propfind>

Response:

<?xml version="1.0"?>
<d:multistatus xmlns:d="DAV:" xmlns:s="http://sabredav.org/ns" xmlns:oc="http://owncloud.org/ns">
	<d:response>
		<d:href>/remote.php/dav/files/admin/test.txt</d:href>
		<d:propstat>
			<d:prop>
				<d:getlastmodified>Thu, 01 Jan 1970 00:20:34 GMT</d:getlastmodified>
			</d:prop>
			<d:status>HTTP/1.1 200 OK</d:status>
		</d:propstat>
		<d:propstat>
			<d:prop>
				<oc:checksums/>
			</d:prop>
			<d:status>HTTP/1.1 404 Not Found</d:status>
		</d:propstat>
	</d:response>
</d:multistatus>

ownCloud version: 10.4.1 (stable)

Start by reproducing the PROPPATCH request against remote.php/dav and inspect the DAV handling of the lastmodified property alongside checksum persistence in the database. Verify the fix by confirming that updating lastmodified changes the timestamp without clearing the file checksum, then repeat the PROPFIND check.