Add support for JSON Marshalling bson.Raw

[SteelPhase]

The default behavior when JSON marshalling a bson.Raw field doesn't print the data represented by the bson.Raw field cleanly. Can a MarshalJSON method be added to bson.Raw which would allow the JSON marshaller to cleanly print a value in bson.Raw?

Current:

{
  "Kind": 4,
  "Data": "{something}"
}

Suggested:

{
  "field": "value"
},

Example MarshalJSON:

// MarshalJSON serializes raw bson into JSON
func (raw Raw) MarshalJSON() ([]byte, error) {
	var rawInterface interface{}

	err := raw.Unmarshal(&rawInterface)
	if err != nil {
		return nil, errors.New("unable to unmarshal BSON data into interface")
	}

	return json.Marshal(rawInterface)
}

[domodwyer]

Hi @SteelPhase

See the comments on the PR - totally up for this addition. I'm going to close this ticket given #108 should land pretty soon.

Thanks!
Dom