fix: HTTP request header is overwritten when user set Content-Type

[hjlarry]

Description

Fixes https://github.com/langgenius/dify/issues/5611 If the user explicity sets the Content-Type field, we should not overwrite it.

Type of Change

• [x] Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

test locally

• [ ] TODO

Suggested Checklist:

• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] My changes generate no new warnings
• [x] I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods
• [ ] optional I have made corresponding changes to the documentation
• [ ] optional I have added tests that prove my fix is effective or that my feature works
• [ ] optional New and existing unit tests pass locally with my changes

[Yeuoly]

Nice fixing~