MediaSet: Call postConstructor correctly
MediaSet: Call postConstructor correctly

After setting parent sessions first and before notifying object creation.

Change-Id: I1ecc67eda7969f3ed4a0b311b7d235b3427518ac

Kurento/kms-core,ESTOS/kms-core,Kurento/kms-core,ESTOS/kms-core,ESTOS/kms-core,Kurento/kms-core,ESTOS/kms-core,Kurento/kms-core