Add functionality not to serialize default values

[ChocPanda]

Add the same functionality as Argonaut Shapeless with regards to Default values

Like upickle, fields equal to their default value are not put in the result JSON object.

case class CC(i: Int = 4, s: String = "foo")

CC().asJson.nospaces == "{}"
CC(i = 3).asJson.nospaces == """{"i":3}"""
CC(i = 4, s = "baz").asJson.nospaces == """{"s":"baz"}"""

"{}".decodeOption[CC] == Some(CC())
"""{"i":2}""".decodeOption[CC] == Some(CC(i = 2))
"""{"s":"a"}""".decodeOption[CC] == Some(CC(s = "a"))