fix(go-http): http quickstart is broken

[guergabo]

Description

The Go HTTP quickstart for pubsub is currently broken. It tries to unmarshal the data the publisher sends into a string. It is not a string, but rather a json string of a map[string]int. I checked with other languages such as python and it seems they also expect a map so went with that. Tested and fixed my issue.

FYI: fixed bug needed to set content-type to application/json. So kinda not a bug cause was originally a string because the http default in golang seems to be text/plain. Regardless, think this is a small improvement for consistency with the other languages (others send regular json payload) and too easy of a bug introduced if they take quickstart and adapt it to there own to send json and get confused like me lol.

Before Bug Fix

After Bug Fix 🤌

Issue reference

Quick bug fix.

Checklist

Please make sure you've completed the relevant tasks for this PR, out of the following list:

• The quickstart code compiles correctly
• You've tested new builds of the quickstart if you changed quickstart code
• You've updated the quickstart's README if necessary
• If you have changed the steps for a quickstart be sure that you have updated the automated validation accordingly. All of our quickstarts have annotations that allow them to be executed automatically as code. For more information see mechanical-markdown. For user guide with examples see Examples.

[paulyuk]

LGTM - thank you for this contribution!