return underlying type for a document for value.Interface()

GODRIVER-548
GODRIVER-529
Change-Id: Ia4364ca7895fa8f15a627ab398db15ed10cb2296

Author: Isabella Siu

bson/element_test.go

@@ -2607,7 +2607,7 @@ func testValidateValue(t *testing.T) {
 						EC.SubDocumentFromElements("barer", EC.Int32("ok", 1)),
 					),
 				),
-				`bson.Document{bson.Element{[string]"foo": "bar"}, bson.Element{[embedded document]"fooer": bson.Reader{bson.Element{[embedded document]"barer": bson.Reader{bson.Element{[32-bit integer]"ok": 1}}}}}}`,
+				`bson.Document{bson.Element{[string]"foo": "bar"}, bson.Element{[embedded document]"fooer": bson.Document{bson.Element{[embedded document]"barer": bson.Document{bson.Element{[32-bit integer]"ok": 1}}}}}}`,
 			},
 			{
 				"nested reader",

bson/value.go

@@ -60,9 +60,15 @@ func (v *Value) Interface() interface{} {
 	case TypeString:
 		return v.StringValue()
 	case TypeEmbeddedDocument:
-		return v.ReaderDocument().String()
+		if v.d == nil {
+			return v.ReaderDocument()
+		}
+		return v.d
 	case TypeArray:
-		return v.MutableArray().String()
+		if v.d == nil {
+			return v.ReaderArray()
+		}
+		return v.MutableArray()
 	case TypeBinary:
 		_, data := v.Binary()
 		return data