Remove optional designation.

Since `payload` and `meta` is generic, user can specify `void` or `undefined` to indicate it does not present.

Removing the optional designation improves type guard.

see `isCustomAction4()` for example

src/index.d.ts

@@ -11,7 +11,7 @@ export interface FluxStandardAction<Payload, Meta> {
    * By convention, if `error` is `true`, the `payload` SHOULD be an error object.
    * This is akin to rejecting a promise with an error object.
    */
-  payload?: Payload;
+  payload: Payload;
   /**
    * The optional `error` property MAY be set to true if the action represents an error.
    * An action whose `error` is true is analogous to a rejected Promise.
@@ -23,7 +23,7 @@ export interface FluxStandardAction<Payload, Meta> {
    * The optional `meta` property MAY be any type of value.
    * It is intended for any extra information that is not part of the payload.
    */
-  meta?: Meta
+  meta: Meta
 }
 
 export interface ErrorFluxStandardAction<CustomError extends Error, Meta> extends FluxStandardAction<CustomError, Meta> {

test/typings-test.ts

@@ -12,7 +12,7 @@ interface MyError extends Error {
   someField: string;
 }
 
-function createCustomAction(payload: CustomPayload): FluxStandardAction<CustomPayload, any> {
+function createCustomAction(payload: CustomPayload) {
   return {
     type: 'custom',
     payload
@@ -31,6 +31,16 @@ function isCustomAction3(action: any): action is FluxStandardAction<void, string
   return isFSA(action) && action.type === 'custom3';
 }
 
+function isCustomAction4(action: any): action is FluxStandardAction<{ message: string }, void> {
+  return true
+}
+
+let action2 = {}
+if (isCustomAction4(action2)) {
+  // type guard infers payload will not be undefined
+  console.log(action2.payload.message)
+}
+
 function reducer(state, action) {
   if (isFSA<CustomPayload, void>(action)) {
     let a: number = action.payload.a;

tsconfig.json

@@ -1,6 +1,8 @@
 {
   "compilerOptions": {
-    "outDir": "tsc-out"
+    "outDir": "tsc-out",
+    "strictNullChecks": true,
+    "target": "es5"
   },
   "files": [
     "test/typings-test.ts"