fix (PersistedEntityInstanceCache): CountOf fixed to support counting interface types

[Ibrahim5aad]

This PR helps in solving #434.

when using and Esent model provider the count of method of the persisted cache throws exception when counting interface types because the implementation supported only getting the corresponding express types of classes.

• [x] Name of this PR is aligned with the name the story.
• [x] It compiles and runs for everybody.
• [x] New code is covered by tests
• [x] All tests run and pass
• [x] Code is readable
• [x] Amount of changes is understandable
• [x] It does what is says it does. I have checked that.