fix Result.isOk

Author: wallymathieu

src/FSharpPlus/Control/Foldable.fs

@@ -425,7 +425,11 @@ type Length =
     static member        Length (x: 'T list          , [<Optional>]_impl: Length  ) = List.length x
     static member        Length (x: option<'T>       , [<Optional>]_impl: Length  ) = if x.IsSome then 1 else 0
     static member        Length (x: voption<'T>      , [<Optional>]_impl: Length  ) = if x.IsSome then 1 else 0
+    #if !NET45
     static member        Length (x: Result<_, _>     , [<Optional>]_impl: Length  ) = if Result.isOk x then 1 else 0
+    #else
+    static member        Length (x: Result<_, _>     , [<Optional>]_impl: Length  ) = match x with | Ok _ -> 1 | _ -> 0
+    #endif
     static member        Length (x: 'T []            , [<Optional>]_impl: Length  ) = Array.length x
     [<Obsolete;CompiledName("Length")>]
     static member        LengthLegacy (x:seq<'T>     , [<Optional>]_impl:Length) = Seq.length   x