added tests for MultiMap methods

tests for containsKey, containsValue and aggregate methods
Prevent these long lines:

```
 int result = multiMap.aggregate(Supplier.all(), Aggregations.integerAvg()).intValue()
assertEquals(50, result)
```

Remember that a test is way more often than it is written. It should acts as documentation and therefore code should be simple.. so that it is very easy to read. You are writing an executable specification... this should not be very technical.. but should be very easy on the eyes.