Document & fix the behavior of isInstanceOf

Author: j-f1

Diff for: Sources/JavaScriptKit/FundamentalObjects/JSObject.swift

@@ -69,9 +69,9 @@ public class JSObject: Equatable {
         set { setJSValue(this: self, index: Int32(index), value: newValue) }
     }
 
-    /// Return `true` if this object is an instance of the `constructor`. Return `false`, if not.
+  /// Return `true` if this value is an instance of the passed `constructor` function.
     /// - Parameter constructor: The constructor function to check.
-    /// - Returns: The result of `instanceof` in JavaScript environment.
+    /// - Returns: The result of `instanceof` in the JavaScript environment.
     public func isInstanceOf(_ constructor: JSFunction) -> Bool {
         _instanceof(id, constructor.id)
     }

Diff for: Sources/JavaScriptKit/JSValue.swift

@@ -155,16 +155,18 @@ public func setJSValue(this: JSObject, index: Int32, value: JSValue) {
 }
 
 extension JSValue {
+  /// Return `true` if this value is an instance of the passed `constructor` function.
+  /// Returns `false` for everything except objects and functions.
+  /// - Parameter constructor: The constructor function to check.
+  /// - Returns: The result of `instanceof` in the JavaScript environment.
     public func isInstanceOf(_ constructor: JSFunction) -> Bool {
         switch self {
-        case .boolean, .string, .number:
+        case .boolean, .string, .number, .null, .undefined:
             return false
         case let .object(ref):
             return ref.isInstanceOf(constructor)
         case let .function(ref):
             return ref.isInstanceOf(constructor)
-        case .null, .undefined:
-            fatalError()
         }
     }
 }