Challenge, SignIn arguments should have params

[Tratcher]

Katana:

void SignIn(AuthenticationProperties properties, params ClaimsIdentity[] identities);
void Challenge(AuthenticationProperties properties, params string[] authenticationTypes);
...
...SignIn(new AuthenticationProperties(), id1, id2);

K:

void Challenge(IEnumerable<string> authenticationTypes, AuthenticationProperties properties);
void SignIn(IEnumerable<ClaimsIdentity> identities, AuthenticationProperties properties);
...
...SignIn(new[] { id1, id2 }, new AuthenticationProperties());

Rumor has it the new run-time will support IEnumerable<T> params so we don't have to use arrays.

Note we'll have to change our parameter order for this, so we should do that soon with or without the IEnumerable<T> feature.