Fix assertMetadata integration test

[MahdiBaghbani]

The whole assertMetadata integration test is currently broken.

reva/tests/integration/grpc/storageprovider_test.go

Line 436 in ccac7ad

Expect(statRes.Info.ArbitraryMetadata.Metadata["foo"]).To(BeEmpty())

We have this access to arbitrary metadata, however it panics, using debugger shows that actually both arbitrary and canonical metadata are nil.

I'll comment them out for now since it's also not working in SM PHP app, we'll have to fix it in future.

[glpatcern]

This is indeed due to a missing implementation of the PHP app, and it's tracked as sciencemesh/nc-sciencemesh#43

So I'm closing this as "duplicate"