Fix handling of falsy data and meta

Falsy data (e.g., `data: 0`) and falsy meta properties aren't preserved
by the middleware, even though FSA allows payload and meta to be
anything, due to the use of `!!data` and `!!meta`.  This commit fixes
this by changing the handling to a more restrictive `!== undefined`.

src/index.js

@@ -44,7 +44,7 @@ export default function promiseMiddleware(config = {}) {
         ...((newPayload === null || typeof newPayload === 'undefined') ? {} : {
           payload: newPayload
         }),
-        ...(!!meta ? { meta } : {}),
+        ...(meta !== undefined ? { meta } : {}),
         ...(isRejected ? {
           error: true
         } : {})
@@ -64,7 +64,7 @@ export default function promiseMiddleware(config = {}) {
         data = payload.data;
       } else {
         promise = payload;
-        data = null;
+        data = undefined;
       }
 
       /**
@@ -74,8 +74,8 @@ export default function promiseMiddleware(config = {}) {
        */
       next({
         type: `${type}_${PENDING}`,
-        ...(!!data ? { payload: data } : {}),
-        ...(!!meta ? { meta } : {})
+        ...(data !== undefined ? { payload: data } : {}),
+        ...(meta !== undefined ? { meta } : {})
       });
 
       /*

test/index.spec.js

@@ -184,11 +184,25 @@ describe('Redux promise middleware:', () => {
       });
     });
 
+    it('pending action optionally contains falsy optimistic update payload', () => {
+      store.dispatch({
+        type: promiseAction.type,
+        payload: {
+          promise: promiseAction.payload,
+          data: 0
+        }
+      });
+      expect(lastMiddlewareModfies.spy).to.have.been.calledWith({
+        ...pendingAction,
+        payload: 0
+      });
+    });
+
     /**
      * If the promise action is dispatched with a meta property, the meta property
      * and value must be included in the pending action.
      */
-    it('pending action does contains meta property if included', () => {
+    it('pending action does contain meta property if included', () => {
       store.dispatch(Object.assign({}, promiseAction, {
         meta: metaData
       }));
@@ -199,6 +213,17 @@ describe('Redux promise middleware:', () => {
       );
     });
 
+    it('pending action does contain falsy meta property if included', () => {
+      store.dispatch(Object.assign({}, promiseAction, {
+        meta: 0
+      }));
+      expect(lastMiddlewareModfies.spy).to.have.been.calledWith(
+        Object.assign({}, pendingAction, {
+          meta: 0
+        })
+      );
+    });
+
     /**
      * The middleware should allow global custom action types included
      * in the config when the middleware is constructed.