Multiple model providers with shared model only first one knows tagset

[reckart]

If we have multiple model providers and only the first one (the one that loaded the
model) will know about the tagset and the others won't.

I think the best way to handle it would be to extend the shared-caching mechanism so
that tagset information is cached along with or parallel to the created resource. 

Cf. issue 504.

Original issue reported on code.google.com by richard.eckart on 2014-11-12 08:45:31