[BUG] Serializer does not use type parameter

[marinusbeck]

Version
LiteDB version 5.0.12 / Windows 11 / .Net framework 4.7

Describe the bug
When serializing a derived class as the base class type it gets serialized as the derived type.

Code to Reproduce

        class BaseClass
        {
            public int A { get; set; }
        }

        class DerivedClass : BaseClass
        {
            public int B { get; set; }
        }

        [Fact]
        public void Serialize_covariant_type_uses_type_parameter()
        {
            var derived = new DerivedClass()
            {
                A = 1,
                B = 2,
            };

            var mapper = new BsonMapper();

            var docA = mapper.Serialize<BaseClass>(derived);
            var docB = mapper.Deserialize<DerivedClass>(docA);

            Assert.True(docB.B == 0); // expected == 0, actual == 2
        }

Expected behavior
The expected would be 0 in the above assert, but is actually 2. This is because the mapper reflects on the input object and not the type parameter when serializing.