eosclient: Handle removal of favorites

[labkode]

When removing the last favorite for a file, the key remains with an empty value:

... other attributes elided ...
user.http://owncloud.org/ns/favorite=""

The problem is due to the way the serialization is performed, using the acl utils:
https://github.com/cs3org/reva/blob/master/pkg/eosclient/eosbinary/eosbinary.go#L507

This fix should be performed both on eosclientbinary and eosgrpc.