Fix issue #630: Unable to override Content-Type header charset

[Vastav1812]

Fix: Prevent automatic charset=utf-8 addition to Content-Type headers

Issue

When sending requests with Content-Type headers like "application/json", the Dart HTTP library automatically appends "charset=utf-8" to the header value. Some APIs reject requests with this charset specification, but users currently have no way to prevent this behavior.

Fixes #630 by giving users control over Content-Type headers.

Changes

• Added useRawContentType boolean field to HttpRequestModel
• Modified HTTP service to respect this setting when setting Content-Type headers
• Added UI checkbox in Headers section labeled "Prevent charset in Content-Type"
• Added tooltip and info dialog explaining the purpose of this option
• Added application-wide setting to control default behavior for all requests
• Added Content-Type header preview in Response Headers section

Implementation Details

• When useRawContentType is true, Content-Type headers are sent without charset
• When false (default), the standard behavior is maintained for compatibility
• New requests inherit the application-wide setting

Testing Instructions

1. Create a POST request with JSON body
2. Observe that by default, Content-Type includes charset=utf-8
3. Enable "Prevent charset in Content-Type" in Headers section
4. Send request and verify Content-Type header is sent without charset
5. Try with application-wide setting enabled to verify it applies to new requests

Related Issues

• Remove charset from "Content-Type" header dart-lang/http#184
• Avoid adding charset to Content-Type header dart-lang/http#1438

[animator]

Please do not modify the PR template. Also, go through the issue and understand it better as this PR is currently not solving it.

[Vastav1812]

So Sorry for that will re-fix the coming issue.

[animator]

In case you are participating in GSoC, it is better that your submit your proposal first as the application window has already opened.