UserService.link called when only using one oauth2 provider

[lukestephenson]

I'm only making use of the the linkedin oauth2 provider and not allowing any other providers. As such, I didn't bother to implement UserService.link as the there is no need to link accounts. I was so confident that this method wouldn't be called, that it throws an Exception if it is called.

However, if the user is already logged in, and they hit http://website/authenticate/linkedin directly, then the UserService.link method is invoked.

I'm not to fussed if the behaviour changes or not, but if it remains as is then I think it's worth updating the UserService documentation to warn that link may be called even if only one provider is used.

[gmethvin]

We definitely should continue to support multiple of each account type. I'm using this functionality to connect multiple Google accounts and combine data from all of them. If you don't want to allow linking multiple linkedin accounts, you need to explicitly implement the link method to make sure that happens.

The documentation should probably make it clear that SecureSocial supports linking any number of accounts, of any type, together into one user.