Add calendar versioning

lib/src/common/github.dart

@@ -21,6 +21,7 @@ class GitHub {
   GitHub({
     Authentication? auth,
     this.endpoint = 'https://api.github.com',
+    this.version = '2022-11-28',
     http.Client? client,
   })  : auth = auth ?? Authentication.anonymous(),
         client = client ?? http.Client();
@@ -29,12 +30,24 @@ class GitHub {
   static const _ratelimitResetHeader = 'x-ratelimit-reset';
   static const _ratelimitRemainingHeader = 'x-ratelimit-remaining';
 
+  @visibleForTesting
+  static const versionHeader = 'X-GitHub-Api-Version';
+
   /// Authentication Information
   Authentication? auth;
 
   /// API Endpoint
   final String endpoint;
 
+  /// Calendar version of the GitHub API to use.
+  ///
+  /// Changing this value is unsupported. However, it may unblock you if there's
+  /// hotfix versions.
+  ///
+  /// See also:
+  ///   * https://docs.github.com/en/rest/overview/api-versions?apiVersion=2022-11-28
+  final String version;
+
   /// HTTP Client
   final http.Client client;
 
@@ -306,6 +319,7 @@ class GitHub {
     }
 
     headers.putIfAbsent('Accept', () => v3ApiMimeType);
+    headers.putIfAbsent(versionHeader, () => version);
 
     final response = await request(
       method,

test/common/github_test.dart

@@ -0,0 +1,26 @@
+import 'dart:io';
+
+import 'package:github/src/common/github.dart';
+import 'package:http/http.dart';
+import 'package:http/testing.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group(GitHub, () {  
+    test('passes calendar version header', () async {
+      Request? request;
+      final client = MockClient((r) async {
+        request = r;
+        return Response('{}', HttpStatus.ok);
+      });
+
+      final github = GitHub(client: client);
+      await github.getJSON(''); // Make HTTP request
+
+      expect(request, isNotNull);
+      expect(request!.headers.containsKey(GitHub.versionHeader), isTrue);
+      final version = request!.headers[GitHub.versionHeader];
+      expect(version, github.version);
+    });
+  });
+}