Repository: Yoctol/bottender (ts)
Issue: Slack interactive events payload isn't parsed correctly
**Describe the bug**

Slack interaction events send back a nested payload, which causes error for event parsing and further handling.
```
{
  "paylaod": "{
    // real payload JSON
  }"
}
```

**To Reproduce**
Steps to reproduce the behavior:
1. set webhook URL for Slack event subscription 
![image](https://user-images.githubusercontent.com/4010549/68449255-6af77500-0221-11ea-8b0a-e76fdc32501e.png)

2. set webhook URL for Slack interaction events
![image](https://user-images.githubusercontent.com/4010549/68449221-413e4e00-0221-11ea-8fc6-93c3e44279b1.png)

3. send a Slack block components message to you slack channel (with buttons or other action components)
e.g.
```
{
	"blocks": [
		{
			"type": "section",
			"text": {
				"type": "plain_text",
				"text": ":wave: Hey David!\n\nWe'd love to hear from you how we can make this place the best place you’ve ever worked.",
				"emoji": true
			}
		},
		{
			"type": "divider"
		},
		{
			"type": "actions",
			"elements": [
				{
				"type": "button",
				"text": {
					"type": "plain_text",
					"text": "Click Me",
					"emoji": true
				},
				"value": "click_me_123"
			}
			]
		}
	]
}
```

**Expected behavior**
Error is caused:
```
TypeError [ERR_INVALID_ARG_TYPE]: The first argument must be one of type string, Buffer, ArrayBuffer, Array, or Array-like Object. Received type undefined
    at Function.from (buffer.js:200:11)
    at SlackConnector.verifySignature (/MyTinyLittleProject/node_modules/bottender/src/bot/SlackConnector.ts:306:35)
    at SlackConnector.preprocess (/MyTinyLittleProject/node_modules/bottender/src/bot/SlackConnector.ts:336:9)
    at server.use (/MyTinyLittleProject/node_modules/@bottender/express/src/registerRoutes.ts:20:52)
    at Layer.handle [as handle_request] (/MyTinyLittleProject/node_modules/express/lib/router/layer.js:95:5)
    at trim_prefix (/MyTinyLittleProject/node_modules/express/lib/router/index.js:317:13)
    at /MyTinyLittleProject/node_modules/express/lib/router/index.js:284:7
    at Function.process_params (/MyTinyLittleProject/node_modules/express/lib/router/index.js:335:12)
    at next (/MyTinyLittleProject/node_modules/express/lib/router/index.js:275:10)
    at jsonParser (/MyTinyLittleProject/node_modules/@bottender/express/node_modules/body-parser/lib/types/json.js:101:7)
    at Layer.handle [as handle_request] (/MyTinyLittleProject/node_modules/express/lib/router/layer.js:95:5)
    at trim_prefix (/MyTinyLittleProject/node_modules/express/lib/router/index.js:317:13)
    at /MyTinyLittleProject/node_modules/express/lib/router/index.js:284:7
    at Function.process_params (/MyTinyLittleProject/node_modules/express/lib/router/index.js:335:12)
    at next (/MyTinyLittleProject/node_modules/express/lib/router/index.js:275:10)
    at /MyTinyLittleProject/node_modules/@bottender/express/node_modules/body-parser/lib/read.js:130:5
```


Pull request description: fix(SlackConnector): use token in payload when received payload JSON …
fix #513

Patch:
diff --git a/packages/bottender/src/bot/SlackConnector.ts b/packages/bottender/src/bot/SlackConnector.ts
index 88b4b50c..213dec22 100644
--- a/packages/bottender/src/bot/SlackConnector.ts
+++ b/packages/bottender/src/bot/SlackConnector.ts
@@ -330,7 +330,12 @@ export default class SlackConnector
       };
     }
 
-    if (!this.verifySignature(body.token)) {
+    const token =
+      !body.token && body.payload && typeof body.payload === 'string'
+        ? JSON.parse(body.payload).token
+        : body.token;
+
+    if (!this.verifySignature(token)) {
       const error = {
         message: 'Slack Verification Token Validation Failed!',
         request: {


Test patch:
diff --git a/packages/bottender/src/bot/__tests__/SlackConnector.spec.ts b/packages/bottender/src/bot/__tests__/SlackConnector.spec.ts
index fbfc09bc..b3599fe6 100644
--- a/packages/bottender/src/bot/__tests__/SlackConnector.spec.ts
+++ b/packages/bottender/src/bot/__tests__/SlackConnector.spec.ts
@@ -408,6 +408,25 @@ describe('#preprocess', () => {
     });
   });
 
+  it('should return shouldNext: true if signature match (token is in payload)', () => {
+    const { connector } = setup();
+
+    expect(
+      connector.preprocess({
+        method: 'post',
+        headers: {},
+        query: {},
+        rawBody:
+          '{"payload":"{\\"token\\":\\"xxxxxxxxxxxxxxxxxxxxxxxxxxx\\"}"}',
+        body: {
+          payload: '{"token": "xxxxxxxxxxxxxxxxxxxxxxxxxxx"}',
+        },
+      })
+    ).toEqual({
+      shouldNext: true,
+    });
+  });
+
   it('should return shouldNext: false and error if signature does not match', () => {
     const { connector } = setup();