No longer polluting the cache with empty results from keys-only queries.

The old code cached empty results when a keys-only query was made.

Here's some basic code that demonstrates the problem:

    g := goon.NewGoon(r)
    q := datastore.NewQuery("BugCheck").KeysOnly()
    var bcs []*BugCheck
    es, _ := g.GetAll(q, &bcs)
    
    e, _ := g.NewEntityById("bug-check-1", 0, nil, &BugCheck{})
    e, _ = g.Get(&BugCheck{}, e.Key)

    if e.Src == nil {
        // Shouldn't be nil, but is because the keys-only query cached it as such
    }