As a use I would like to assert Should().BeJsonSerializable<T>

[borismod]

[TestMethod]
public void SerializableClass_ShouldBeJsonSerializable()
{
    var obj = new SerializableClass();
    obj.Should().BeJsonSerializable<SerializableClass>();
}

[rikrak]

I was going to add this but saw that there was already a PR[1] with a request to base it on the release-5.0 branch - Is this still an active branch or should I start from master

[1] https://github.com/fluentassertions/fluentassertions.json/pull/2/commits/54432fdebc39e8ac52f5c9a0a30c08bdab01bbcf

[dennisdoomen]

If it's a non-breaking change, you can use master. I removed the other two branches.