Remove usage of `should` in test method names betterspecs.org/#should

It seems like the test uses a lot of `should` instead of just stating what it does, and what the purpose of the test is.

betterspecs.org/#should is properly the best description to why should is disallowed. Also its a best practice to not use should when speccing .
but the spec does not check that it gets serialized at all. What it checks is that it is of type SerializableInterface. So IMO, ``it_is_serializable`` is still the best name for what it does