Enable resolution of unprefixed dataset IDs

[dhermes]

This started in #528 before an existing bug existed.

[dhermes]

Porting comment from @jgeewax on related issue (#475):

This isn't possible with the current dataset_id situation requiring s~ or e~ prefixes in
the key protobuf. Moving to Datastore Future since we don't know for sure when
this will be fixed.

[yjkogan]

+1 to this. I had an issue with ndb.StructuredProperties related to this where app engine couldn't access an entity's structure properties b/c one of them was a key with _app=MY_APP instead of _app=s~MY_APP.

[dhermes]

@yjkogan Comment makes me realize I should link to googleapis/google-cloud-datastore#59 in this issue.

@yjkogan You are better off just using s~MY_APP as your dataset ID.

We'd like to help along people who don't realize the prefix exists, but since you do, it's no big deal to use. Does that make sense?

[yjkogan]

@dhermes Yeah I just set s~MY_APP as my dataset id. It was an easy workaround. Just +1ed since I'm in favor of making that distinction invisible to users of this library.

[dhermes]

Great!

[dhermes]

This is made moot by v1beta3, follow #1288 for progress towards that.