Fixes #7393

The automatic URL-encoded serialization fails when the Content-Type header uses initial uppercase letters. This is because headers.normalize() was called without the formatHeader parameter.

Changes:

• Export the formatHeader function from AxiosHeaders.js
• Pass formatHeader to headers.normalize() in transformData.js

Summary by cubic

Normalize headers with formatHeader in transformData to fix case-sensitive Content-Type handling. Restores automatic URL-encoded serialization when header casing varies.

Description

• Summary of changes
  • Export formatHeader from lib/core/AxiosHeaders.js.
  • Pass formatHeader to headers.normalize() in lib/core/transformData.js.
• Reasoning
  • headers.normalize() without formatHeader failed to match mixed-case Content-Type, causing urlencoded serialization to be skipped.
• Additional context
  • Fixes Automatic serialize urlencoded fails for "Content-Type" with initial upper case #7393.
  • No breaking changes or migrations.

Testing

• No tests added in this PR.
• Add regression tests:
  • Ensure urlencoded serialization works with Content-Type, content-type, and other case variants.
  • Verify no change for already-lowercase headers.

^{Written for commit 2b0e2cd. Summary will update on new commits.}