Fix test module code style

[mamadaliev]

Hello, @Patreon!
Please merge my changes if you think these changes are correct.

1. Optimize imports in PatreonAPITest and PatreonOAuthTest classes.

// from:
import org.mockito.Matchers;
import org.powermock.api.mockito.PowerMockito;
import static org.mockito.Mockito.mock;
import static org.mockito.Mockito.verify;
import static org.mockito.Mockito.when;

// to:
import org.mockito.*;

2. Fix assertion to simplify in PatreonAPITest class.

// from:
assertEquals(false, campaign.isChargedImmediately());

// to:
assertFalse(campaign.isChargedImmediately());

3. Fix property access modifier to private.

// from:
PatreonOAuth oauth = ...

// to:
private PatreonOAuth oauth = ...

4. Remove "Exception" from "testAuthorizationURL" throws list.

// from:
public void testAuthorizationURL() throws Exception ...

// to:
public void testAuthorizationURL() ...

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