bugfix:fix the bug that middleware will return illegal http body when http body is empty (#737)

If the http body is empty. The httpBody function will return HTTP header content from index 3. This is unexpected.
Function caller also cannot tell whether the httpbody is empty or not.

middleware/middleware.js

@@ -335,7 +335,12 @@ function deleteHttpHeader(payload, name) {
 }
 
 function httpBody(payload) {
-    return payload.slice(payload.indexOf("\r\n\r\n") + 4, payload.length);
+    let bodyIndex = payload.indexOf("\r\n\r\n");
+    if (-1 != bodyIndex){
+        return payload.slice(bodyIndex + 4, payload.length);   
+    } else {
+        return null;
+    }
 }
 
 function setHttpBody(payload, newBody) {
@@ -710,6 +715,12 @@ function TEST_httpBody() {
     if (body != "hello") {
         fail(`'${body}' != 'hello'`)
     }
+
+    const exampleInvalidPayload = "Invalid HTTP Response by Network issue";
+    let invalidBody = httpBody(Buffer.from(exampleInvalidPayload));
+    if (invalidBody != null) {
+        fail(`'${invalidBody}' != 'null'`)
+    }
 }
 
 function TEST_setHttpBody() {