Use values that follow CanonicalHeaderKey in Header checks

This is just a stylistic nit (cc @laurentsenta @hacdias for visibility)

https://pkg.go.dev/net/http#CanonicalHeaderKey

Our tests should have already canonical format, to ensure users who are in rush and copy&paste blindly will use normalized version.

For example:

```diff
- Header("X-IPFS-Path")
+ Header("X-Ipfs-Path")

- Header("ETag")
+ Header("Etag")
```