json serialization with FieldSerializer fails to read a class with option members. Here are two examples to reproduce the error:

----

import net.liftweb.json._ 
import net.liftweb.json.Serialization._ 
class TestClass{ 
  val field : Option[String] = Some("i am a string") 
} 
implicit val formats = DefaultFormats  + FieldSerializer[TestClass]() 
read[TestClass](write(new TestClass))

Throws Exception: net.liftweb.json.MappingException: Parsed JSON values do not match with class constructor

----

import net.liftweb.json._ 
import net.liftweb.json.Serialization._ 
class TestClass{ 
  var field : Option[Int] = None 
} 
val t = new TestClass 
t.field = Some(5) 
implicit val formats = DefaultFormats  + FieldSerializer[TestClass]() 
read[TestClass](write(t))

Throws exception: net.liftweb.json.MappingException: unknown error

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