Change `content-type` header for POST requests from "text/plain;charset=UTF-8" to `application/json`

[arjshiv]

Describe the bug If the request content type header is not set to application/json , common API frameworks like express with body-parser cannot parse it

To Reproduce Steps to reproduce the behavior:

1. Set up CopilotTextArea
2. Send a request to an express server with body-parser middleware enabled [ really common for JSON ]
3. Try and parse the body with req.bod
4. req.body is empty

Expected behavior I expect

Screenshots If applicable, add screenshots to help explain your problem.

** Browser (please complete the following information):**

• Universal

Additional context

• Sending JSON as json is a standard convention, this could break not just Express but most other frameworks.

[iamtouha]

@ataibarkai I would like to work on this issue.

[ataibarkai]

Great @iamtouha , let me know if you need any support

[iamtouha]

Certainly! Could you please specify which files I should be looking at to apply the changes.

[nsvoriginals]

Is this bug done?

[ElvinPero]

@ataibarkai Created a PR regarding this issue. https://github.com/CopilotKit/CopilotKit/pull/149