Potential conformance issue with encoding/json

[epelc]

Hi Quest,

As part of reading through many docs I just noticed a potential issue you might have. UnmarshalJSON is supposed to be a no-op when it encounters null.

https://golang.org/pkg/encoding/json/#Unmarshaler

By convention, to approximate the behavior of Unmarshal itself, Unmarshalers implement UnmarshalJSON([]byte("null")) as a no-op.

See second part after || of this line. I believe it should be removed or at least noted.
https://github.com/qhenkart/objectid-go/blob/master/objectid.go#L155

Was very hidden in std lib docs. Think I have to fix a spot or two in my projects also.