Summary by cubic

Adds documentation showing how to preserve specific HTTP header casing in axios using case presets (seeding with undefined) and AxiosHeaders.concat. This helps when integrating with servers that require exact header case.

Description

• Summary of changes
  • README: new “Preserving a specific header case” section with examples.
  • Advanced docs: updates to header-methods and headers with guidance and links.
• Reasoning
  • While headers are case-insensitive, some servers behave otherwise; AxiosHeaders keeps the first-seen case, so we document a reliable pattern to control it.
• Additional context
  • Shows presets via api.defaults.headers.common and composition with AxiosHeaders.concat.

Testing

• Docs-only changes. No tests added or needed.

^{Written for commit 2ee674d. Summary will update on new commits.}