JSON constructors create values that don't correspond to the original schema

[Tarmil]

The constructors provided by JsonProvider can create values in a way that doesn't correspond to the original schema. In particular:

1. Optional fields are serialized as null rather than absent field:

   type A = JsonProvider<"""[{"a":1,"b":2}, {"a":3}]""">
   
   let a = A.Root(a = 1, b = None).JsonValue.ToString()
   // Expected: {"a":1}
   // Actual:   {"a":1,"b":null}
   

2. Decimal fields inferred from string are serialized as number:

   type B = JsonProvider<"""{"a":"1.4"}""">
   
   let b = B.Root(a = 3.1m).JsonValue.ToString()
   // Expected: {"a":"3.1"}
   // Actual:   {"a":3.1}
   

[cboudereau]

Same problem here for option an json strings (double quotes surrounded values)

[joshuapassos]

Same problem on #1245