_jkey for undefined or missing value should return None not Some

[wallymathieu]

Expected behavior

When you read non existing keys using _jkey should return None.

#r "nuget: Fleece.FSharpData, 0.10.0"
open Fleece
open Fleece.FSharpData
open FSharp.Data
open Fleece.FSharpData.Lens
open FSharpPlus
open FSharpPlus.Lens

JsonValue.Parse( "{ \"b\": 200}" ) ^? (_jkey "a" );;
// val it: JsonValue option = None

Actual behavior

The _jkey returns Some for undefined or missing keys.

#r "nuget: Fleece.FSharpData, 0.10.0"
open Fleece
open Fleece.FSharpData
open FSharp.Data
open Fleece.FSharpData.Lens
open FSharpPlus
open FSharpPlus.Lens

JsonValue.Parse( "{ \"b\": 200}" ) ^? (_jkey "a" );;
// val it: JsonValue option = Some null

#r "nuget: Fleece.NewtonsoftJson, 0.10.0"
open Fleece
open Fleece.Newtonsoft
open Fleece.Newtonsoft.Lens
open FSharpPlus
open FSharpPlus.Lens

JsonValue.Parse( "{ \"b\": 200}" ) ^? (_jkey "a" );;
// val it: JsonValue option = Some <seq>