GODRIVER-2766 Support inherited defaultDocumentType

[prestonvasquez]

GODRIVER-2766

Summary

The DecodeValue method in the struct codec creates its own DecodeContext to decode the underlying value. This approach can cause issues with ancestor selection, as the defaultDocumentType constraints on empty interfaces are not enforced.

In addition to updating the struct.DecodeValue's DecodeContext, this PR parallelized a subset of the decoder tests.

Background & Motivation

GODRIVER-2407 defined a bug wherein the empty interface codec (EIC) will interpret embedded data to be the same type as the parent (or "ancestor") data. In the case of 2766:

item := &struct {
    A map[primitive.ObjectID]interface{} `json:"foo" bson:"foo"`
}{}
 
jsonData := "{ \"foo\" : {\"63ea2082b1ce23422a8bf9ca\" : { \"bar\" : \"\" } } }"
 
err = json.Unmarshal([]byte(jsonData), item)
if err != nil {
    fmt.Println("json unmarshal err", err.Error())
}

The embedded data is the map {bar: ""} but the EIC is interpreting it as a "map[primitive.ObjectID]interface{}". See PR #951 for the work-around solution and the long-term solution. This PR is necessary because the work-around is currently invalid for the struct codec, a bug uncovered during the investigation of 2766 ticket.

[matthewdale]

Nice find! Looks good 👍