fix: correct response headers TypeScript types to use lowercase keys [Hacktoberfest]

[abhishekmaniy]

• Branch: fix/response-headers-lowercase-typing
• Single line change: Updated header names to lowercase
• Zero breaking changes: All existing code continues to work
• Create PR at: https://github.com/abhishekmaniy/axios/pull/new/fix/respons
  e-headers-lowercase-typing

Suggested PR Description:

Summary

Fix response headers TypeScript types to use lowercase keys matching
runtime behavior.

Problem

Response headers are normalized to lowercase at runtime (as documented),
but TypeScript types incorrectly used Title Case headers, causing
type/runtime mismatches.

Solution

Change CommonResponseHeadersList to use lowercase header names:

• 'Server' → 'server'
• 'Content-Type' → 'content-type'
• 'Content-Length' → 'content-length'
• etc.

Test case

const response = await axios.head(url);
// ✅ Now properly typed and has runtime value
const length = response.headers['content-length'];
// ❌ Title Case headers are undefined at runtime
const titleCase = response.headers['Content-Length']; // undefined

Fixes the TypeScript/runtime mismatch reported in the issue.

This minimal fix aligns the TypeScript definitions with axios's documented
and actual runtime behavior!

[jasonsaayman]

we actually need to solve for any case to work... i will keep this open sorry about the premature closing