fix(config_test): use canonical HTTP header names in expectedHeaders

Go's http.Header canonicalizes header names (e.g., X-GitHub-Api-Version
becomes X-Github-Api-Version). The test's unexpected header check was
failing because it compared canonical names against non-canonical keys
in the expectedHeaders map.

Signed-off-by: Mike Ball <[email protected]>

Author: mdb

github/config_test.go

@@ -390,7 +390,7 @@ func TestPreviewHeaderInjectorTransport_RoundTrip(t *testing.T) {
 			existingHeaders: map[string]string{},
 			expectedHeaders: map[string]string{
 				"Accept":               "application/vnd.github.v3+json",
-				"X-GitHub-Api-Version": "2022-11-28",
+				"X-Github-Api-Version": "2022-11-28",
 			},
 			expectRoundTripCall: true,
 		},
@@ -406,7 +406,7 @@ func TestPreviewHeaderInjectorTransport_RoundTrip(t *testing.T) {
 			},
 			expectedHeaders: map[string]string{
 				"Accept":               "application/json,application/vnd.github.v3+json",
-				"X-GitHub-Api-Version": "2021-01-01,2022-11-28",
+				"X-Github-Api-Version": "2021-01-01,2022-11-28",
 			},
 			expectRoundTripCall: true,
 		},