fix uint32 encode.

[xtutu]

When encode a uint32 type, it will got NumberLong() in bson.

for example

type Data struct {
	Number uint32 `bson:"Number"`
}

func TestBson(t *testing.T) {
	data := &Data{
		Number: 100,
	}
	byteData, _ := bson.Marshal(data)
	fmt.Println(byteData)

	var after = &Data{}
	bson.Unmarshal(byteData, after)
	fmt.Printf("%d\n", after.Number)
}

Output is :
[21 0 0 0 18 78 117 109 98 101 114 0 100 0 0 0 0 0 0 0 0]

According to this description: http://bsonspec.org/spec.html
18 = "\x12", it means 64-bit integer type

so, the more right output should be:
[17 0 0 0 16 78 117 109 98 101 114 0 100 0 0 0 0]

[divjotarora]

Hi @xtutu,

Thank you for your contribution to the Go driver. We recognize that this is an issue, but we don't think this PR is the correct solution. For your use case specifically, you can call bson.MarshalWithContext with an EncodeContext that has minSize set to true. We are also working on adding options to codecs so that users who want minSize behavior can easily create a BSON Registry to enable it.