client: use canonical names for HTTP Headers

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

client/request.go

@@ -238,14 +238,14 @@ func (cli *Client) addHeaders(req *http.Request, headers headers) *http.Request
 	// Add CLI Config's HTTP Headers BEFORE we set the Docker headers
 	// then the user can't change OUR headers
 	for k, v := range cli.customHTTPHeaders {
-		if versions.LessThan(cli.version, "1.25") && k == "User-Agent" {
+		if versions.LessThan(cli.version, "1.25") && http.CanonicalHeaderKey(k) == "User-Agent" {
 			continue
 		}
 		req.Header.Set(k, v)
 	}
 
 	for k, v := range headers {
-		req.Header[k] = v
+		req.Header[http.CanonicalHeaderKey(k)] = v
 	}
 	return req
 }