Revert usage of http#CanonicalHeaderKey

[jpkrohling]

// Extract X-Scope-OrgID
key := http.CanonicalHeaderKey(orgID)

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

The canonicalization converts the first letter and any letter following a hyphen to upper case; the rest are converted to lowercase.

So that will convert X-Scope-OrgID to X-Scope-Orgid based on my reading @jpkrohling @rlankfo
We see 500s in Dev for every request - can this be changed / rolled-back?

Originally posted by @leveyjam in #60 (comment)