rpc: canonicalize header keys in WithHeaders

WithHeaders assigned header keys verbatim, unlike WithHeader which
uses Header.Set and canonicalizes keys. Normalize keys to avoid
duplicate headers with different casing.

rpc/client_opt.go

@@ -92,7 +92,7 @@ func WithHeaders(headers http.Header) ClientOption {
 	return optionFunc(func(cfg *clientConfig) {
 		cfg.initHeaders()
 		for k, vs := range headers {
-			cfg.httpHeaders[k] = vs
+			cfg.httpHeaders[http.CanonicalHeaderKey(k)] = vs
 		}
 	})
 }