fix for unwanted fields coma separators between omitted Option[T] fields

I discovered that the serialization of class case with several (more than two in succession) Option[T] fields produce invalid JSON object with unwanted comma separators.

Example:
```scala
case class OptionalValues(a: Option[String], b: Option[String], c: Option[String])

write(OptionalValues(None, None, Some("stringValue"))) // => { , "c": "stringValue"}
```