add unit tests for listBuckets()

lib/minio.dart

-/// Support for doing something awesome.
-///
-/// More dartdocs go here.
 library minio;
 
 export 'src/minio.dart';
+export 'src/minio_errors.dart';
 
 // TODO: Export any libraries intended for clients of this package.

test/minio_dart_test.dart

+import 'package:minio/minio.dart';
+import 'package:test/test.dart';
 
 void main() {
-  // group('A group of tests', () {
-  //   Awesome awesome;
+  group('listBuckets', () {
+    test('listBuckets() succeeds', () async {
+      final minio = _getClient();
 
-  //   setUp(() {
-  //     awesome = Awesome();
-  //   });
+      expect(() async => await minio.listBuckets(), returnsNormally);
+    });
 
-  //   test('First Test', () {
-  //     expect(awesome.isAwesome, isTrue);
-  //   });
-  // });
+    test('listBuckets() fails due to wrong access key', () async {
+      final minio = _getClient(accessKey: 'incorrect-access-key');
+
+      expect(
+        () async => await minio.listBuckets(),
+        throwsA(
+          isA<MinioError>().having(
+            (e) => e.message,
+            'message',
+            'The Access Key Id you provided does not exist in our records.',
+          ),
+        ),
+      );
+    });
+
+    test('listBuckets() fails due to wrong secret key', () async {
+      final minio = _getClient(secretKey: 'incorrect-secret-key');
+
+      expect(
+        () async => await minio.listBuckets(),
+        throwsA(
+          isA<MinioError>().having(
+            (e) => e.message,
+            'message',
+            'The request signature we calculated does not match the signature you provided. Check your key and signing method.',
+          ),
+        ),
+      );
+    });
+  });
 }
+
+/// Initializes an instance of [Minio] with per default valid configuration.
+///
+/// Don't worry, these credentials for MinIO are publicly available and
+/// connect only to the MinIO demo server at `play.minio.io`.
+Minio _getClient({
+  String endpoint = 'play.minio.io',
+  int port = 443,
+  bool useSSL = true,
+  String accessKey = 'Q3AM3UQ867SPQQA43P2F',
+  String secretKey = 'zuf+tfteSlswRu7BJ86wekitnifILbZam1KYY3TG',
+  String sessionToken = '',
+  String region = 'us-east-1',
+  bool enableTrace = false,
+}) =>
+    Minio(
+      endPoint: endpoint,
+      port: port,
+      useSSL: useSSL,
+      accessKey: accessKey,
+      secretKey: secretKey,
+      sessionToken: sessionToken,
+      region: region,
+      enableTrace: enableTrace,
+    );