[#456] Enable calling real implementation on extensions method from jdk8

Enable calling real implementation on extensions method from jdk8
Additionally change add validation checking whether callRealMethod is invoked on abstract methods.
In fact resolving issue could be achieved by removing this validation because when someone do next thing:
1: when(someInteface.abstractMethod()).thanCallRealMethod();
2: someInteface.abstractMethod()
Exception will be throwned on second line informing that there were attempt to invoke abstract method