Fix empty queryParameters.payload in the request sent from Dialogflow to the fulfillment webhook

[mekpavit]

WHY

Normally, the queryParameters.payload, which is put in projects.agent.sessions.detectIntent, should be shown in the request that is sent from Dialogflow to the fulfillment webhook. The document states that the payload CAN BE BOTH ProtoStruct or JSON objects, but, when the JSON objects was implemented, no payload is received by the webhook. (session.detectIntent)

After searching for the solution, this problem was once mentioned in this StackOverflow question. It seems that converting the queryParameters.payload from JSON objects to Struct solves the issue. So, this pull request is proposed to implement this solution to the dialogflow-integration server.

WHAT

• Add jsonToProto function which used to convert JSON objects to StructProto (the code was copied from matthewayne repo).
• Add the unit test for jsonToProto function (the mock input and expected output used in the unit test for protoToJson function are used here as well).
• Implement jsonToProto function to the queryParams.payload field in DialogflowSessionClient.constructRequest (this change will convert payload from JSON object to ProtoStruct and solve the mentioned issue).

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

:memo: Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it! and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

ℹ️ Googlers: Go here for more info.

[mekpavit]

@googlebot I signed it!

[googlebot]

CLAs look good, thanks!

ℹ️ Googlers: Go here for more info.

[tyhu-google]

mekpavit@, thanks for sending your pull requests. Dialogflow team will review and verify it. We will merge the request if it all looks good.