api [nfc]: Factor out helper for making auth HTTP header

Author: chrisbobbe

lib/api/core.dart

@@ -37,16 +37,20 @@ abstract class ApiConnection {
   Future<String> post(String route, Map<String, dynamic>? params);
 }
 
+MapEntry<String, String> authHeader(Auth auth) {
+  // TODO memoize
+  final authBytes = utf8.encode("${auth.email}:${auth.apiKey}");
+  return MapEntry('Authorization', 'Basic ${base64.encode(authBytes)}');
+}
+
 /// An [ApiConnection] that makes real network requests to a real server.
 class LiveApiConnection extends ApiConnection {
   LiveApiConnection({required super.auth});
 
   Map<String, String> _headers() {
-    // TODO memoize
-    final authBytes = utf8.encode("${auth.email}:${auth.apiKey}");
-    return {
-      'Authorization': 'Basic ${base64.encode(authBytes)}',
-    };
+    final Map<String, String> result = {};
+    result.addEntries([authHeader(auth)]);
+    return result;
   }
 
   @override