fix: refactor requestContextPlugin.spec.js to use response headers

[shawshankkumar]

Fixes #175

I replaced the json body with value in headers. I found this to be the best approach. if anyone has any better solutions, happy to implement that as well.

Checklist

• [x] run npm run test and npm run benchmark (can't find script benchmark)
• [x] tests and/or benchmarks are included
• [ ] documentation is changed or added
• [x] commit message and code follows the Developer's Certification of Origin and the Code of conduct

[alfonsograziano]

#177 is a possible alternative approach. Not sure what is the best one :)