CascadeSave should sync

[wxf4150]

this commit fixes #36

[wxf4150]

DocumentBase fields should omitempty; then the sub struct can used as bongo.Find param.

type User struct {
	bongo.DocumentBase `bson:",inline"`
	UserName     string `example:"wxf"`
	Password string `json:"-" example:"111111"  bson:",omitempty"`
	Phone string ` bson:",omitempty"`
	Email string ` bson:",omitempty"`
}
u:=new(User)
err:=bgoUser.FindOne(bson.M{"username":uname},u)  

err:=bgoUser.FindOne(User{UserName:uname},u)  // if DocumentBase fields  bson omitempty;  then this line will work too.

[coveralls]

Coverage decreased (-1.02%) to 84.064% when pulling 6721120 on wxf4150:master into 761759e on go-bongo:master.

[coveralls]

Coverage decreased (-1.02%) to 84.064% when pulling 6721120 on wxf4150:master into 761759e on go-bongo:master.

[coveralls]

Coverage decreased (-1.02%) to 84.064% when pulling 6721120 on wxf4150:master into 761759e on go-bongo:master.

[coveralls]

Coverage decreased (-1.02%) to 84.064% when pulling 6721120 on wxf4150:master into 761759e on go-bongo:master.