relaxed restrictions on OAuth callbacks

src/test/java/org/scribe/builder/ServiceBuilderTest.java

@@ -46,9 +46,9 @@ public class ServiceBuilderTest
   }
 
   @Test(expected=IllegalArgumentException.class)
-  public void shouldNotAcceptAnInvalidUrlAsCallback()
+  public void shouldNotAcceptNullAsCallback()
   {
-    builder.provider(ApiMock.class).apiKey("key").apiSecret("secret").callback("example.com").build(); 
+    builder.provider(ApiMock.class).apiKey("key").apiSecret("secret").callback(null).build(); 
   }
 
   @Test