NSAssert failure when refresh_token is not used and access_token is expired

I have found an issue if refresh_token is not used by OAuth2 server and access_token is expired or is about to expire (10 seconds to expire).

The issue causes application to crash on NSAssert in NXOAuth2Client.m, because method refreshAccessTokenAndRetryConnection requires to pass access_token together with refresh_token.

According to OAuth2 specification, the refresh_token is optional.

Related code where assertion fails if refresh_token is empty:
```objc
- (void)refreshAccessTokenAndRetryConnection:(NXOAuth2Connection *)retryConnection;
{
    // <cut> 

    if (!authConnection) {
        NSAssert((accessToken.refreshToken != nil), @"invalid state");
```


Yeah, we should get rid of those ;-)