Error when session not found

[enmanuelr]

http://www.gorillatoolkit.org/pkg/sessions#Registry.Get

If my interpretation of the gorilla docs is correct, when the session is not found, the store should create a new session and return it. An error should not be generated if the session is not found.

https://github.com/antonlindstrom/pgstore/blob/master/pgstore.go#L249

If the query on this line returns sql.ErrNoRows, it's a good idea to generate a new session and return it.

[mfzl]

I just spent an embarrassing amount of time trying to figure what the issue was. Turns out it was not ignoring the error when no rows were found.

@antonlindstrom please have a look at #24