A previously deleted quad cannot be re-added when using GaeDatastore

[gkontos]

Description

When using datastore, a deleted quad cannot be re-added. This happens if the code is deleting and adding quads in order to update. This error will be encountered if ignore_duplicate is set to true when using Datastore.

Steps to reproduce the issue:
q := quad.Make(quad.IRI("ID"), quad.IRI("schema:name"),quad.String("Original Name"),nil)
qs.AddQuad(q)

Delete the quad :
qs.RemoveQuad(q)

Add the quad again :
q := quad.Make(quad.IRI("ID"), quad.IRI("schema:name"),quad.String("Original Name"),nil)
qs.AddQuad(q)

Received results:
The quad will not be retrieved by an iterator.

Expected results:
The quad should be retrieved by an iterator.

Output of cayley version or commit hash:
v0.6.1

Environment details:
Gaedatastore

** Potentially misleading user research **
Line 186 of graph/gaedatastore/quadstore.go starts a nested if statement which will bypass setting keep = true. With keep = false, the node doesn't get an additional ID for quad.Added in the updateQuads() function. Line 250 of graph/gaedatastore/iterator.go will then skip the quad in the Next() function.

[barakmich]

Fantastic bug report. User research seems solid 👍