Add unit tests for ProductType sync

[heshamMassoud]

Description

As a consequence of this discussion, the productType sync module should also be covered by unit tests.

[heshamMassoud]

Also refer to #302

[ahmetoz]

Currently, we have test cases for the product types, https://github.com/commercetools/commercetools-sync-java/blob/master/src/test/java/com/commercetools/sync/producttypes/ProductTypeSyncTest.java