UserStore.FindAsync Incorrect Query Logic

[kingdango]

Thanks for the work on this. I found a bug while I was "borrowing" your existing work and thought I would help. I don't plan to use this library verbatim at this point so instead of submitting a commit I will just comment below:

The UserStore FindAsync implementation is incorrect. Although the query will probably match the user most of the time it will yield the wrong user in some cases. It is also not as performant as the suggested change.

user = _users
    .FindOne(Query.And(Query.EQ("Logins.LoginProvider", login.LoginProvider), 
        Query.EQ("Logins.ProviderKey", login.ProviderKey)));

Should be changed to use ElemMatch which actually compares two values of the same element of an array instead of two distinct properties of any of the array values.

var user = _users.FindOne(
                Query<TUser>.ElemMatch(i => i.Logins, i => Query.And(
                    Query<UserLoginInfo>.EQ(loginRow => loginRow.LoginProvider, login.LoginProvider),
                    Query<UserLoginInfo>.EQ(loginRow => loginRow.ProviderKey, login.ProviderKey))));

I haven't tested the change but I do know ElemMatch is what should be used for a query on multiple properties of a single array element. Wanted to let you know before I moved on. Thanks again for the work on this!