resolved Unable to override Content-Type header

[connectwithpankaj]

PR Description

Fixed Content-Type Header Handling:

Previously, Content-Type was only set if requestModel.hasContentTypeHeader was false.
Now, it explicitly checks if requestModel.bodyContentType.header.isNotEmpty before setting the header.
Improved Multipart Request Handling:

Previously, only text fields and files were handled.
Now, it ensures proper header management and optimizes file uploads.
Better Header Management:

Ensures Content-Length is dynamically set when sending request bodies.
Enhanced Error Handling:

Improved exception handling for unexpected errors.
More robust cancellation detection (wasRequestCancelled).
GraphQL Request Handling Fix:

Ensures GraphQL requests always set Content-Type: application/json.
Performance Optimization:

Uses a Stopwatch to track execution time and ensure it stops properly.

Related Issues

• Closes #

Checklist

• I have gone through the contributing guide
• I have updated my branch and synced it with project main branch before making this PR
• I am using the latest Flutter stable branch (run flutter upgrade and verify)
• I have run the tests (flutter test) and all tests are passing

Added/updated tests?

• Yes
• No, and this is why: please replace this line with details on why tests have not been included

OS on which you have developed and tested the feature?

• Windows
• macOS
• Linux

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This PR does not resolve the issue.