Ensure leading "v" for facebook graph version

[fritzmg]

If the leading v is missing, the generated authorization link will be wrong when using this in combination with the facebook/graph-sdk for example (e.g. https://www.facebook.com/2.11/dialog/oauth?client_id=… instead of https://www.facebook.com/v2.11/dialog/oauth?client_id=…).

It should be ensured, that the leading v is always present when saving or at least an exception should be thrown, if it is missing.

Discovered via @vossmedien