Fix GetBSON() method usage

[bozaro]

Original issue

You can't use type with custom GetBSON() method mixed with structure field type and structure field reference type.

For example, you can't create custom GetBSON() for Bar type:

struct Foo {
	a  Bar
	b *Bar
}

Type implementation (func (t Bar) GetBSON() ) would crash on Foo.b = nil value encoding.

Reference implementation (func (t *Bar) GetBSON() ) would not call on Foo.a value encoding.

After this change

For type implementation func (t Bar) GetBSON() would not call on Foo.b = nil value encoding.
In this case nil value would be seariazied as nil BSON value.

For reference implementation func (t *Bar) GetBSON() would call even on Foo.a value encoding.