Switching datastore cursors to URLsafe base64 encoding.

[theacodes]

Fixes getting-started-python#8

[tseaver]

Given that our code only sends the cursor as a member of a protobuf passed as the body of a POST request (with Content-Type: application/x-protobuf), I don't know why the urlsafe_ encoding variants should matter: the cursor never appears as an item in a URL path or query string, nor even in a body encoded via www-form-urlencoded/multipart-formdata. Do we know whether the back-end uses the urlsafe_ variant to encode the cursor?

@jonparrott Were you able to reproduce GoogleCloudPlatform/getting-started-python#8 before this patch, and then have it not fail with the patch?

[dhermes]

I don't know why the urlsafe_ encoding variants should matter

For users creating URLs to page through search results with the cursors

---

Do we know whether the back-end uses the urlsafe_ variant to encode the cursor?

The backend uses bytes. The only reason we encode bytes as base64 is for our end-users and we decode it when bringing it back in to a protobuf (e.g. if the user gets a base64 encoded cursor from a next_page() and then they use that as an end cursor).

[theacodes]

Inside of gcloud-python, the cursor is never passed around in a URL or query string, but in user applications it's a common pattern:

def list():
    cursor = request.params.get('cursor', None)
    query = ds.query(kind='Book', order=['title'])
    it = query.fetch(limit=limit, start_cursor=cursor)
    entities, more_results, cursor = it.next_page()
    return "Next page: {}".format(url_for('list', cursor=cursor))

(abridged)

Returns something like:

/list?cursor=CkUKEwoFdGl0bGUSChoIdGhlb2RvcmUSKmoVc35iaWd0YWJsZS1zZmFuLXRlc3QxchELEgRCb29rGICAgIC+goMKDBgAIAA=

which fails due to the + present.

[theacodes]

Thanks, @tseaver. I'll be on the lookout for the next release so I can update getting-started-python.