fix bug of another comma added

previous code will add comma if the field is empty when is not first, like 
```
{"account":"1120","name":"nick",,"department":"fsaf","role":1,"privilege":32,,"created_at":1489226500}
```

feature_reflect_object.go

@@ -1058,12 +1058,12 @@ func (encoder *structEncoder) encode(ptr unsafe.Pointer, stream *Stream) {
 	stream.WriteObjectStart()
 	isNotFirst := false
 	for _, field := range encoder.fields {
-		if isNotFirst {
-			stream.WriteMore()
-		}
 		if field.omitempty && field.isEmpty(ptr) {
 			continue
 		}
+		if isNotFirst {
+			stream.WriteMore()
+		}
 		field.encode(ptr, stream)
 		isNotFirst = true
 	}
@@ -1097,4 +1097,4 @@ func (encoder *emptyStructEncoder) encodeInterface(val interface{}, stream *Stre
 
 func (encoder *emptyStructEncoder) isEmpty(ptr unsafe.Pointer) bool {
 	return true
-}
+}